Weather effects appear in caves and structures in 1.17.1 worlds loaded in 21w43a

Wither Skeletons can only spawn on Nether Bricks

Withers do not move down to follow the player when the wither's health is half or lower

Icebergs go all the way to ocean floor.

Weather can sometimes be incorrect within a biome

Copper Ingots are no longer grouped in the crafting table

Burning mobs can still destroy powder snow when gamerule mobgriefing is disabled

Large dripstone structures can be generated outside the caves