IsThisMandatory

In case it helps people stumbling upon this after trying to use those weighted pressure plates :
Using the behavior that vines/ladders ignore cramming, these pressure plates are usable. Any part of the mob hitbox can intersect the pressure plate and trigger it, while the center of the bottom face of the mob's hitbox is what must be inside the grid aligned cube containing the ladder/vine.

This can be considered a "I have read the terms and conditions" mechanic 🙂

Confirmed (procedure described above in a random new survival world) :
Stepping up, fall damage in pre-4
Stepping up, no fall damage in pre-5

I have just taken a look at all the announced 1.15.x changes. The boat change does indeed seem most likely, good guess !

I doubt the boat fix is its origin. Afaik it was due to the server accumulating player falling speed (as if the boat was not blocking the player from falling, and the player was teleported back up to their correct position, albeit without losing speed, every tick), while the client didn't.

It's possible, but that would imply that it would be related via something about player velocity, and that stepping up negates the accumulated falling speed. I doubt that stepping up things is done via velocity changes.
We may then think of a scenario where it's done by some form of teleportation, meaning that such teleportations reset the falling velocity, which feels like an odd fix to the boat thing (why fix the speed part instead of the client/server desync ?)

Thanks for the speculation 🙂