The Scourge are my favorite villain faction in World of Warcraft. Indeed, they've had a pretty huge influence on my fantasy tastes in the 20 years I've played this game. Going into WoW, perhaps thanks to watching through all of Buffy the Vampire Slayer and Angel in my freshman year, I was all about demons as fantasy monsters to fight (in the Buffyverse, vampires are sort of undead but are technically a demon possessing a dead body, though the demon retains all the memories of the person that had died. Also, demons in that universe aren't inherently evil, though vampires are except in the rare cases, like two of them, where they have their human soul returned to them).
I had even kind of avoided playing an Undead character simply because I sort of assumed Undead players were all edgelords. And then I made one and I had a dark realization - that I'm a bit of an edgelord myself.
I will say that the utter menace that the Vanilla Plaguelands held got lost a bit in the Cataclysm revamp. The Western Plaguelands are more of a battleground between Alliance and Horde while the Eastern Plaguelands, though they do remain a hotbed of Scourge corruption, give us a somewhat more innocent story of the Paladin Pals and Fiona's caravan going around the zone. Back in Vanilla, both zones felt oppressive and hostile, and Light's Hope chapel truly felt like a tiny bastion of safety in the midst of a hostile land. A few details reinforced that: for one thing, Light's Hope didn't have any of the fortifications it does now, and was just a single little church building mostly in ruins that the Argent Dawn was posted up at (they even had a neutral and interactable Scarlet Crusade representative there, desperate for any help). Also, in the original level ranges, the Eastern Plaguelands was one of the highest-level zones in the game (maybe the highest? I think you wouldn't start getting quests there until 55 or so).
Icecrown is also a zone that I think radiates menace better than just about any other we've visited. Possibly the Antoran Wastes or The Maw give it a run for its money (I have so many thoughts on the Maw, primarily that the things I liked most about it were what a lot of players complained about, and I felt it got ruined in 9.1). The Scourge were a constant presence in Wrath of the Lich King, with at least some presence in each zone (maybe not Crystalsong Forest? But there was barely anything there).
But the Scourge is not what it once was. Originally created as a cannon fodder to clear the way for the Burning Legion's invasion in the Third War, Ner'zhul and Arthas managed to break away from the yoke of the Legion following Arichimonde's defeat on Mount Hyjal, and when Arthas became the Lich King, we had the period of "peak Scourge" that ran from the end of the Frozen Throne expansion for Warcraft III to the end of Wrath of the Lich King. (Bizarrely, their only real presence on Kalimdor post-Third War was in Razorfen Downs, where they were secretly corrupting and empowering the Quillboar - which comes as a real surprise when you quest through the Barrens given that there's no real hint of undeath elsewhere in the zone).
I think it's also notable that not all undead in WoW is Scourge - there are plenty of ghosts (particularly of ancient Night Elves who perished in the War of the Ancients or the Sundering) but we even see zombies and even Abominations pop up in places like Duskwood, where the corruption is probably coming more from Karazhan or just, you know, ancient curses and old crimes.
But still, the Scourge was the only real example we had of a disciplined and organized effort to convert the entire world into undead.
Arthas' death led to the crowning of Bolvar Fordragon as the third and final Lich King. In theory, this was necessary as a means to keep control over the Scourge and prevent them from running rampant over Azeroth. Now, I've often wondered if that was actually any worse than having someone like Arthas commanding them in an organized manner. While I do think the intent by the game's writers was that this would be somehow worse, it's easy enough to headcanon that, less than a warning of how bad it could get, Terenas' warning was more like "hey, you have a huge opportunity here to force the Scourge into just staying here frozen in Icecrown and not bothering people if someone with a strong enough will becomes Lich King." Bolvar's reign largely accomplished that. While he directed the Knights of the Ebon Blade to do some deeply questionable (ok, maybe just bad) things in their class campaign in Legion (the attack on Light's Hope - to be fair, I think Bolvar anticipated that it would be Darion and not Tirion who would be raised as the fourth horseman, but it did get a lot of paladins killed, and the attack on the Ruby Sanctum was also pretty indefensible,) for the most part, the Scourge ceased to be much of a threat.
He didn't have a short leash on everyone, though: we do see in those Cataclysm-era Eastern Plaguelands quests that there are still Scourge forces and the Cult of the Damned trying to cease the healing of the land and corrupt it further, like their attempt to make Gidwin into a Death Knight. But these splinter factions of Scourge are clearly not very powerful. There's a Crypt Lord who claims to be the new leader of the Scourge whom we take out in a fairly simple quest there.
Shadowlands complicated the Scourge narrative a lot: while the Shadowlands had been established as early as Wrath (Death Knight get their Acherus Deathcharger by journeying into the Shadowlands,) the actual structure of the plane was elaborated on quite a lot - initially, the Shadowlands were presented lik the Emerald Dream, as a kind of parallel plane to the mundane world, a bit like how in D&D the Feywild and Shadowfell are both reflections of the Prime Material plane. Indeed, my interpretation was (and still is, sort of) that when we die in-game and are in our spirit form to go reclaim our body, that "ghost world" we're in is The Shadowlands. The presentation of the plane in the expansion of the same name does seem to refute this, but it might be similar to D&D's Ethereal Plane, where there's the "Border Ethereal" that overlaps the Prime Material and can be used as a means to pass unseen and even through objects like walls, and the "Deep Ethereal," which is an endless misty expanse where one can find portals to other inner planes like the Feywild, Shadowfell, or the Elemental Planes. So, maybe the "ghost world" is the "Border Shadowlands" while the place we visited with Bastion and Revendreth is the "Deep Shadowlands." Just like in D&D, it's simpler to get into the Border region than it is to go to the deep region of the plane.
Anyway, beyond having this "realm of death," it also established god-like figures and a cosmic power of Death separate from the demons of the Burning Legion or the Void that the Undead draw their power from. Structurally, the Scourge as an army is a weird facsimile of the forces of Maldraxxus, though ironically, as gross and monstrous as Maldraxxus appears, it's not actually bent toward evil (well, at least not originally).
Shadowlands introduced a ton of new elements to the lore, and while I like the idea of making Death into this power with its own god-like figures in and of itself, certainly some of this could be handled more gracefully - especially as it was implied that the Jailer had secretly been behind nearly the entirety of Warcraft Lore (such as implying that his agents had even orchestrated Sargeras' fall from grace).
Indeed, the retcon of the Nathrezim being creatures of the Shadowlands, rather than the Fel, originally, was particularly jarring for a number of reasons. First off, their homeworld of Nathreza was already established, so then making them the creation of Sire Denathrius felt like it was giving a second origin of their names and natures. But it also makes it really weird that the Nathrezim were introduced as the Legion's handlers for the Scourge - if the Nathrezim were always secretly loyal to Death, wouldn't Mal'ganis and Detheroc and all those guys actually be in favor of the Lich King breaking away from the Legion? Sure, they're master infiltrators and can keep up a cover story, but this feels just so very weird and stinks to high heaven of retcon (which is 100% is).
The Shadowlands as an expansion had some very cool ideas, but there was so much messiness in incorporating it into the broader WoW lore that I'm almost inclined to think that Sylvanas' line about "the Shadowlands are not what they seem" could very easily be Blizzard giving itself license to retcon anything they presented in that expansion.
While I think the Jailer was a character who could have been super interesting but wound up being really dull (the reveal that he was the original Arbiter was a cool origin story, but his actual characterization and indeed just his physical design from a visual standpoint were... I didn't like them,) I do think the idea of "Domination Magic" coming out of the Shadowlands and forming the basis for the Scourge's power is an interesting one. And I do really like the idea that the Scourge's power is fundamentally different from the demonic power of the Legion.
But let's return to Azeroth:
At the start of Shadowlands, Sylvanas tears the Helm of Domination apart, effectively making the existence of a Lich King impossible. The Scourge is no longer held back by Bolvar, who is also left as effectively just a Death Knight now. We did spend the pre-launch event for Shadowlands in Icecrown fighting Scourge, but while the Scourge remains a general threat (even in Midnight, while questing with Arator we do have to fight some Scourge attacking Light's Hope) they really seem downgraded.
Now sure, this could just be a matter of emphasis. The Scourge were no less a danger during the events of Burning Crusade, but we were dealing with other issues, and so they didn't figure prominently in the story. Perhaps post-Shadowlands, the Scourge has become a bigger problem than it was under Bolvar, but we're just not seeing it as much because we've got dragons and void things to deal with.
One element we haven't really explored a lot is that there's a new helm: in Shadowlands, to counter the effect of the Jailer's Domination Magic (which incapacitates us at the end of the Torghast raid and forced Anduin into an Arthas-like role as pseduo-Death Knight,) the remnants of the Helm are used to forge the Crown of Wills, which acts as an almost diametric opposite to the Helm by magically reinforcing free will. Currently worn by Pelagos, who has become the new Arbiter (my main is a protection paladin, and had Pelagos as his soulbind, so I like to say that my character is canonically best friends with God,) it seems like its effects are tied more to the Shadowlands themselves, and likely embody a kind of reformation of the place where souls can decide where they want to go.
But does this have an effect on the Scourge? We really don't know. While the Forsaken have had free will for a while (though there were times from about Cataclysm through Shadowlands where it didn't actually seem that way - Night Elves raised by Sylvanas' Val'kyr during the War of Thorns pre-BFA event very suspiciously joined the Horde's efforts immediately after being raised - perhaps a hint of Sylvanas' alignment with the Jailer) what if that's starting to be a broader thing for the Scourge?
After all, it was Illidan's attempt to shatter the Frozen Throne that first allowed the Forsaken to break away. Surely the destruction of the Helm of Domination would lead to an even greater effect. But, of course, while the Forsaken did regain free will, plenty of them chose to do some nasty and evil shit.
While I assume The Last Titan is going to focus far more on the Ulduar/Storm Peaks/Titan stuff in Northrend, the fact that we're going back there means that we must have some interaction with the Scourge. I wouldn't be surprised if they play a major role in the middle raid tier of the expansion, similar to how we've gotten a kind of break from the Devouring Host while we deal with Ula'tek and her whole deal.