21W42 feedback: Don't remove underground spawns

I’ve spent a long time searching for cave spawn seeds where I spawn underground, but in this latest snapshot that was marked as a “bug” and removed entirely. As it was, It’s hard enough finding a cave spawn where you respawn underground too, so this is very disheartening. Please at least make it an option like min/max spawn height or just a very rare occurrence instead of being lost entirely.