Improved Mob Spawning & Keeping Old Mob Farms Working

With the lowered down world limit, many efficient mob farms built before are going to be slower by a lot. This is because currently the game bases spawning chance on the distance from highest non-air block to the lower bound of the world.

That's why I suggest it should instead base it on the distance from highest non-air block to the lowest non-air block. If you also left old terrain as it is, this would keep rate of the old farms the same, while at the same time it would work the same for new terrain as it does now. I don't think it's a problem to keep the air layer below old chunks, as it wouldn't require any updates to old chunks and players could enjoy building something below bedrock (if their world was created before 1.17). If you also went through the effort of breaking a lot of bedrock under your farm, it could increase the rates, especially for structure-based farms as they are limited by the bounding box.

One last thing to keep old farms working is to not change where slimes can spawn as it currently is, ranging from y-64 to y40.