Place soul sand in front of a two-block high doorway, and try to walk through. You will be able to do so.
This change was probably made to address MC-62170, which prevented people from walking through doorways with carpets in them. However, it also removed one-way doors, which have been around pretty much forever, have been used in numerous contraptions, and have been marked as "works as intended" multiple time on this bug tracker.
Comments 2
Well, in 1.7, a doorway would also act as a one-way door if there was a lower-half trapdoor on the bottom block of the doorway, so I don't think that the feature had to do with the player walking into another block, just with the player walking upwards. When carpets started to have a non-zero height, they also became a problem for people who didn't want their doors to have the one-way effect (which is why MC-62170 was created). However, it seems that Mojang decided to fix MC-62170 by removing one-way doors altogether.
Darn. This is disheartening. I don't think it has anything to do with MC-62170 though. The player is no longer "jumping" up to the next block.