Because of the new world depth, hostile mobs now spawn at a much slower rate in any y-coordinate below y=128 than they did before 21w06a. This is because, despite there being up to twice as many spots for mobs to spawn, there is the same mob cap as before. It is now common to go through a cave system without seeing any hostile mobs even on hard difficulty. This makes the survival aspect to the game considerably easier.
In 21w15a, the world height has been reverted, but this can be reproduced with the datapack to make the world depth go down to -64.