Slime can't spawn on shroomlight.Shroomlight can't do damage to mob and it can prevent player open chest.It can be charged by a redstone repeater.Why can't mob spawn on it?
Jack o'Lantern is very similar to shroomlight but mob can spawn on Jack o'Lantern.
Mob can't spawn on fully charged respawn anchor either.
Looking at the code (
BlockBehaviour.java), it looks like blocks that emit a light level of 14 or greater do not allow mob spawning (regardless of the individual mob's light requirements) except for Jack o'Lanterns and redstone lamps, which override this behavior.