Spellcasters of the Monster Manual

 One of the most notable redesigns we got in Mordenkainen Presents Monsters of the Multiverse was the transformation of spellcasting statblocks. The Evoker from Volo's Guide to Monsters, for example, has the Sculpt Spells feature of the playable subclass, and then just has the ordinary spell slot loadout of a 12th level wizard, with a bunch of prepared spells. Their only "action" is to whack with a quarterstaff - something you'll probably never do as a DM, as it causes this CR 9 creature to deal 3 damage a round.

The Evoker Wizard, the MotM replacement, does have spells, and some even that do damage (like Lightning Bolt,) but they don't use spell slots and their primary meat-and-potatoes attack action is a multiattack that allows them to shoot three Arcane Bursts - spell attacks that deal 25 damage on a hit.

The point here is that a creature like this is way easier to run. They can absolutely shoot off a lightning bolt or two, but when in doubt, the DM can just have them shoot these Arcane Bursts and the damage output should be appropriate or the CR (skewing a little more toward damage output given having less survivability - fitting as a wizard).

I really prefer this sort of stat block. Spells and spell slots work very well as a player-facing mechanic, but if you want a conclave of evil wizards for your party to fight as a DM, you don't want to have seven different wizards, each with their own spell slots to track, and with a dozen different spells to pick from on each of them.

This, presumably, is the model moving forward for D&D spellcasters. And we're likely to see all the spellcasters in the Monster Manual transformed in a similar way.

Lots of creatures that cast spells have things like innate spellcasting, but that's clearly not being gotten rid of. Instead, it's the player-facing spell slot system that looks like it will be changing.

This post isn't really meant to go deep on the philosophy or design - I kind of just want a place to list all of these creatures to refer back to. So let's take a look, in alphabetical order, at all the creatures in the Monster Manual who use spell slots to cast spells.

Acolyte

Androsphinx

Arcanoloth

Archmage

Bone Naga

Cult Fanatic

Death Knight

Drow Mage

Drow Priestess of Lolth

Druid

Flameskull

Guardian Naga

Gynosphinx

Hag Covens (special)

Kuo-toa Archpriest

Kuo-toa Whip

Lich

Lizardfolk Shaman

Mage

Mind Flayer Arcanist (a variant option)

Mummy Lord

Orc Eye of Gruumsh

Priest

Sahuagin Priestess

Spirit Naga

Vampire Spellcaster (a variant option)

And there you have it. Of the 450 monsters (give or take) in the Monster Manual, these 25 (again, ish - Covens covers three types are the ones that would warrant a redesign.

Tons of other monsters can cast spells, but they do this via Innate Spellcasting, which in most cases presents such options as supplementary to either weapon attacks or bespoke abilities.