Version: 3.0.1
ModLoader: Fabric
Expected behaviour: Nothing happens
What happens: Game/world (server) crashes trying to generate a feature of any kind
Cause: Mod adds minecraft:air to the #minecraft:stairs and #minecraft:slabs block tags (and others, but those don't show any issue), which in some dynamic structures, makes it look for the facing, type or waterlogged states, which it cannot find. Thus ensues the crash.
Expected fix: remove the minecraft:air from any block/item tag that shouldn't include it.

This issue occurs any time a structure or feature fetches a random instance from one of those two tags, and it happens to fall on the air values.