So, usually when I'm playing around trying to evaluate the general damage potential or whatnot of various classes and builds, I do my amateurish attempt at coming up with averages - averaging out damage, but also hit and critical strike chances.
However, sometimes, raw data can be helpful as well.
I wanted to get a sense of how well the 2024 version of the Monk, with the change from Deflect Missiles to Deflect Attacks, compares with other classic tank characters, namely the Barbarian and the heavily-armored Fighter/Paladin.
What I did was imagine typical examples of each character class, built to be somewhat tanky (though I didn't give the Fighter - what I'll be calling the Fighter/Paladin just to save on letters typed - the Defensive fighting style or take into account things like the Sap mastery). The Barbarian and Fighter in both of these cases is using a Shield.
I made a couple assumptions: first that, via point buy, our Monk is starting off with +3s to both Wisdom and Dexterity so that their AC at level 3 (when they get Deflect Attacks) is 16. Our Barbarian has a +3 to Strength and Con, but took a slight hit to Dex with a mere +2 so as not to have to utterly tank all mental stats, so with a shield their AC is 17. Our Fighter, of course, could have basically any stats and it wouldn't matter, starting with chain mail and a shield for an AC of 18.
I used the Knight stat block from the recent free Scions of Elemental Evil adventure, which has several monsters from next year's new Monster Manual. The Knight has a +5 to hit, and gets two attacks with a Greatsword (or Heavy Crossbow, but we're assuming melee here) and deals 2d6+3 slashing plus 1d8 radiant damage on a hit.
Here's how things would go:
The Monk gets one reaction each time to deflect an attack that hits them, reducing the damage by, at this level, 1d10+6. The Barbarian is assumed to be Raging, so they will take half damage from the slashing, though full damage from the radiant. The Fighter then will take all the damage when an attack beats their AC.
I then simulated ten rounds of attacks against the characters - naturally, this would be way longer than any fight was likely to last, but the idea was that we would hopefully smooth out the hit chance and damage to be closer to the average.
The Monk was crit 3 times and hit 5 times, which was a high number of crits (in 20 attacks, they'd normally take a single crit) and a low number of hits (the Knight had a 50% combined hit/crit chance, but only landed blows 40% of the time). Some of these hits were fully reduced to 0, though a lot of damage slipped through because two of the three crits landed on the same turn, meaning while one got reduced to just 3 damage, the other came through as a full 25. The Monk wound up, over 10 rounds, taking 58 damage.
The Barbarian was hit only 5 times, and never crit, which meant things skewed pretty low for them. Also, I seemed to roll terribly on the d8 of radiant damage, only once rolling above a 4. The Barbarian wound up taking a total of 40 damage.
The Fighter, despite having the highest AC of the group, wound up getting crit once (which is right in line with probability) and hit 6 times (the Knight had a 40% combined hit/crit chance, so even in this case, it was slightly less damage than they should have taken). And the total damage taken? 123.
Yeah, this surprised me a bit, though I suppose that the difference in AC is pretty small, while the ability to mitigate damage is non-existent for our Fighter/Paladin. Now, my main experience playing a Fighter has been as an Eldritch Knight, and as long as he had a spell slot left, his AC was much higher (he had gotten Adamantine Plate and a +1 Shield, along with the Defensive fighting style, which meant that against a Knight like this, they would only be able to hit him on a critical, which would then be downgraded in damage.) But I think that if you don't have some way to push your AC up into the stratosphere, the lack of further damage mitigation actually makes you, weirdly, not as good of a tank (again, having a Sap weapon might make up for this, though the Barbarian can also take advantage of that).
Then, because my Wizard in my longest-running campaign is level 7, I tried this out at that level as well. Here, the Monk is getting higher Dexterity, which increases their AC to 17 and also this and their level means their Deflect Attacks is now mitigating 1d10+11 damage. The Barbarian actually doesn't get really any further mitigation (as they probably boosted Strength with their ASI) while I'm allowing the Fighter to have upgraded to Splint armor (I think 8 is usually the level where you can finally afford plate armor).
Again, we're just using actual rolled dice, and so this isn't 100% indicative of how the average results would go:
The Monk took 8 hits and 1 Crit, and after Deflect Attacks, they wound up taking a total of 56 damage. (They probably should have taken a bit less as I was rolling pretty low on the d10 for Deflect Attacks). Still, the number of hits and crits here matched the expected numbers - they took 8 hits and 1 crit.
The Barbarian took 7 hits and no crits, which was a bit low, as they should have taken one more hit and one crit in there somewhere. They wound up taking a total of 56 damage, exactly equal to the Monk, but arguably with better luck (which means I give this to the Monk, actually).
The Fighter took 5 hits and no crits, again, both one fewer than expected. However, across those 5 hits, they took 75 damage.
Now, there is one factor we haven't talked about, which is that Monks will generally have fewer hit points than Fighters and Paladins, and them fewer HP than Barbarians. I think the Monk, in this scenario at least, will wind up taking the least damage of them all, but the Barbarian is still a viable and possibly still supreme tank.
But, my major takeaway is that if you want a Monk as a front-line tank, it's actually very viable. The key, here, though, is that we're also in an environment where each of these characters is getting attacked only twice per round. While Barbarians and Fighters/Paladins (even an Eldritch Knight, because the Shield spell will last until the start of your next turn) will mitigate damage proportionately regardless of how many attacks are made, the Monk really prefers to face a single foe with one or two bit attacks, thus allowing them to use DA on proportionately more of those attacks. Against a big swarm of enemies, they're not as effective at tanking that damage.
Also, the Monk really does need to reserve their reaction, while the others are free to use theirs when appropriate.
Still, I think a Monk tank is quite capable.