There have been no changes made to the mob spawning algorithm in the 1.17 cave update. This means that the singleplayer mob cap of 70 is expected to distribute across the surface and 128 blocks of caves. Additionally, mobs prefer to spawn more in the lower parts of the world. This effectively makes anywhere except the deepslate caves an extra-easy mode, and even the deepslate caves are almost mob free due to the many other caves mobs can spawn in.
Additionally, mobs prefer to spawn more in the lower parts of the world.
I see people claiming this a lot but it is simply not true and can only be a misunderstanding of the fact that the game chooses a random altitude (with a uniform distribution) between the bottom of the world and the top of the highest spawnable block, meaning that mobs will spawn more frequently at a given layer if the ground is lower but the spawn rate for the lowest layer is always the same as the highest layer (ignoring the fact that sky light, including at night, lowers the success rate of spawn attempts). Compared to the mob cap the actual spawn rates also barely matter; in TMCW mobs only spawn 1/4 as often per chunk as in vanilla but it has no problem keeping the mob cap full even when flying around in Creative unless I use a preset like Tunneler's Dream (230 blocks deep).
Of course, mobs will still be distributed over a greater volume so the density will be lower so that could still be an issue, but it also depends on how cave density compares to before (I seriously doubt that they have made the world the Swiss cheese it was before 1.7, and TMCW has about twice the underground volume of vanilla 1.6.4, so 1.17 would have to have the average density of 1.6.4 over 128 layers to be equivalent), and I have no problem with a lack of mobs, then again, TMCW does reduce the spawn/despawn radius from 128 to 96 blocks and the no-spawn zone around the player from 24 to 16 blocks, more than doubling the density of mobs around the player for the same spawnable surface area (it should also be noted that the volume of a cave increases by the cube of the width while surface area is only squared).
For an extreme case, I averaged only 117 mob kills per play session in my "triple height terrain" world, which had the equivalent of about 3.5 times the current depth from sea level to cave lava level and 4 times the underground volume of vanilla 1.6.4 with vanilla mob spawning, compared to an average of 303 in my first world (vanilla 1.6.4 underground and spawning) and 365 in TMCWv4 (modded as mentioned above; when adjusted for session length I averaged about 10% more mob kills per hour so the underground and spawning changes mostly cancel out).
Either way, I'd hope that Mojang considers this; as mentioned above, the greater underground volume in TMCW is canceled out by a reduction in the (de)spawning range of mobs, which isn't even noticeable (aside from mob farms, which I don't consider) since mobs are only visible up to 80 blocks away, less for smaller mobs, and even Ghasts only attack from up to 64 blocks (they latch onto a target from up to 100 blocks but do not shoot until they are within 64 blocks). As a bonus, this allows the minimum practical render distance to be lowered from 10 to 8 (mobs are only ticked if a 5x5 chunk area around them is loaded; 128 blocks = 8 chunks and 96 blocks = 6 chunks. That said, TMCW limits the minimum server-side view distance to 8 (vanilla 1.6.4 is fixed at 10) so lower render distances have no negative impact on mob spawning or random block updates (+/-7 chunks from the player), which IMO is the best way to fix MC-2536 (the fact that this is still unresolved does not bode well).
With new world gen, there is a whole lot more spawnable surface to go around. In my snapshot survival, I am definitely seeing less mobs on the surface at night than in earlier versions.
Same thing with caves/mineshafts/strongholds - fewer mobs in any given area. Also, woodland mansion had the usual complement of illagers, but the 'supporting cast' was definitely lacking, compared to what it is usually like.
To download the other ones you need to make a folder in the versions folder for minecraft and put the client and JSON file for the versions in there. They all need to be named the same aside from file extensions. Once you do that, you will be able to choose that version when making a new profile with the minecraft launcher.
They surely already know everything but It's not the time to think about this yet, we have to see the final cave generation first, the new mobs generating below 0 and only then it's time to tweak mob generation
Ray works tested mob spawning in the new snapshot, all the mob farms are about half as effective in 1.17, so that shows that mob spawning is seriously nerfed due to 1.17 and thus there would be less mobs above Y0