• Elytra doesn't work
• Fall slowly
• Climb always

Appears to have been introduced in 20w51a.

Checking the diff between 20w49a and 20w51a, the most suspicious change is a tags refactor. However that would seem odd because the climbable tag was introduced in 1.16, not 1.14. Maybe the synthetic tags got corrupted somehow...

The climbable tag was supposed to contain ladder, vine and scaffolding. Scaffolding was introduced in 1.14, so when connecting to 1.13.2 and below this should be removed from the tag. However, multiconnect never did this, and instead left the scaffolding block instance in the tag. This worked fine until 20w51a, when the tag system is now no longer able to distinguish between the removed scaffolding block and the air block (which is the default block in the registry). The fix was to remove the block from the tag when the server doesn't know the block.