The mobs no longer swim up (or drown) in water. I waited a few minutes at my dual skeleton farm and checked after if they were still spawning (in case spawners broke). The spawners were working, as you can see skeletons do spawn however they don't swim up and I waited a few minutes and they don't drown.

[Image in attachments]

I haven't seen reports (yours or the community's, not even in the changelogs) of a world upgrade stopping mob farms from being farms (since I figure most people haven't tried that yet).

I have seen other YouTubers already using this design building it in 1.13 (in a new world for themselves) which works, however after a world upgrade from 1.12.2 this style of mob trap no longer functions currently in this snapshot.

Came back to share this: https://youtu.be/30Qy6Kg6RtU?t=6m48s - PythonMC creates one that uses water for skeletons to swim up, I tested that was as well (2 wide, water diagonal adjacent) and for some reason Skeletons still don't swim up in my world.