Mob elevators using soul sand to make bubble columns not possible due to mobs (skeletons and all forms of zombies):

1) No longer swimming up when their head is in water.

2) Water mechanics prevent water from pushing mobs into the bubble column, so now they sit at the bottom (I shouldn't need to give an example of a water elevator normally used for skeleton or zombie farms in previous versions of Minecraft for comparison, but if necessary, will add a screenshot of one built in Minecraft 1.12.2), as water flows out of the bottom of the bubble column instead of into it (if using a stair block to create the lift into the bubble column).

3) Since Mobs do not slide across Ice or Packed Ice, if that is used next to the soul sand, the mobs simply stop on the ice, then may or may not decide to move into the bubble column.

4) This may cause Zombies from a Zombie Spawner to become Drowned.

5) Though the water may flow out of the top Bubble Column block, it does not push mobs out of that block like a water source block flowing would, so they get stuck there, as well (this may be another bug that needs to be reported).