This is a classic example of a change in both philosophy and design.
And, for me, a very welcome one.
Up to 3rd edition, NPCs had to conform to PC creation.
To a certain extent, see below. Yes, it's mostly true in 3e, and a real pain it was, forcing you to create very complex NPCs who did not end up using even half of the abilities allocated to them.
A NPC was simply a NON PC. At some point, an NPC could have some special abilities given to him by a god, powerful devil or whatever. But basically, they were following the same rules. It took something exceptional to get additional tricks.
The thing is that I don't restrict NPCs to "PC races", never have. I have always had Monsters used as NPCs, as you mention adding a few things, spells mostly, often at will. It's only 3e who forced me to codify this, and who forced all races/monsters to add class levels to increase the power, so that CR could be computed.
4th edition did the opposite. NPC were not following the PC rule at all. Each NPC was unique and different with its own stat block. Just like PCs had their own "unique" powers that were more or less identical but name differently accross the classes. We all know how 4ed ended...
But still (and although, for many reasons, it was not my cup of tea), it sparked a lot of interesting ideas, some of them made it into 5e, others like minions were dropped, but overall the harvest was not that bad at all.
5ed went back to NPC follow mostly the same rule. Spell casters especially with spell slots, the same spells but not the same level progression and a few other minor stuff.
This is only if you look at human NPCs spellcasters. But if you look at non spell-casters, there is not a hint of level progression, they just have some abilities, some of them mirroring that of classes, others not at all (look at things like "parry", for example, which is not an ability that PCs have at all, although they have some abilities of the same kind).
A blend of 3rd and 4ed if you want. But basically, they were following the same rules as the PC for the most part.
I really don't agree. Yes, some have spell lists and these are more or less conform to PC spell lists, but not always, and others have at will spell. But this is only for the casters, and for the "human-like", but I've created
tons of other NPCs with specific rules (some of them mirroring PC classes, but some not, and in any case nothing like the progression), see here. And in all cases, they have far fewer abilities than PCs, so they really don't have a class (and again it's a good thing, makes the NPC much simpler to run).
Now, we have a full reverse toward 4ed style opponents but unfortunately, PCs have a shared spell slot system across the board. I don't want my 5ed play like 5ed for the PCs but like 4ed for the monsters. The blend was brilliant but now I feel betrayed. It is as if they had given me what I wanted until I had accepted this way of doing it long enough to push 4ed style down my throat.
Nothing prevents you from using the previous versions, nothing forces you to go to the new format. Just consider them different NPCs/Monsters.