In the 21w07a snapshot, dirt is said to stop generating below y 0, but mineshafts still generate dirt rooms as part of their structure below y 0.
Coords: /tp @s -612.30 -49.09 -113.43 -574.20 36.15
Dirt in negative y coordinates
Dirt Spawns under y=0 in an interesting form.
Dirt below y=0
Mineshaft generation broken
Dirt square near mineshaft
Strange grass floor in the cave
mineshaft generates dirt below y=0 in Caves&Cliffs snapshots
Coal ore can generate below Y=0 when attached to a fossil
Mineshaft dirt rooms above y=0 no longer generates with dirt