Some biome exclusive mob spawn rates are seemingly reduced. I suspect this is due to the oddities that 3D biomes have introduced. 
It's complicated to explain, but basically if you were to go to a Taiga for example you wouldn't see nearly as many foxes as before.

How to replicate

Using /kill for foxes in a taiga in 21w37a will range from "no entity was found" to "Killed 4 entities." Of course this varies but in general it seems to favor lower numbers. Likewise using it in 1.17 will immediately yield numbers such as 18 or 21.

The same can be said for parrots in jungles. Numbers drop from 25 in 1.17 to 10 in 21w27a.
I'm sure a large collection of tests in various seeds/locations would yield better results, and give a better idea of how MUCH this affects mob spawning. However even just exploring on foot in survival, it "feels" like there is less. 

Mobs affected

Could relate to MC-236689, though in that bug Pandas aren't spawning at all. Foxes and parrots and the like still do spawn, just in lower numbers.

I don't know the exact extent of what biome exclusive mobs this affects if not all of them, but I also suspect ocelots, llamas, axolotls, and wolves. Completely untested for hostile mobs like Husks/Strays but could apply to them as well.

It also seems like Drowned are affected by this. Killing any naturally generated Drowned from ocean ruins, then letting them spawn back yields significantly less results than former versions.

What I expected to happen:

Biome-specific mob spawning to act as usual and spawn a decent amount of mobs in its specific environment.