Hostile mobs do not seem to spawn below y:1, however wandering traders still spawn down to y:-59. Not sure if this is related to the integration of the warden into future versions but thought it worth a mention. Further findings from my investigation revealed that no matter what, villagers cannot survive a 120 block drop. Lol.
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Correction, below y:0 no hostile mobs spawn and unable to spawn wither.
Iron golem and snow golem spawn with no issue down to y:-59, haven't tested further, will update if I do.