I've done a bit of testing on this on a realm. I can consistently crash a realm causing it to revert to the previous save with this method. This can be exploited by people wanting to revert progress / lost items, etc. I have a feeling this can be exploited to dupe items as well as it seems sometimes the save that is loaded is slightly different than what had occurred, like player position, or for example if an entity is killed, but when "going back in time", other things are reset but the entity is still missing, etc. One player in the realm claims to have duped a bow this way, but not everyone agrees it was a dupe.
We've not done extensive testing on this additional finding, but it seems that the parrot-donned player may revert to a default alex/steve skin and when going through portals without the parrot, switch back and forth between alex and steve. (basically, surgeon general warning: going back in time may cause you to swap genders)
Process:
1. get a parrot on your shoulder
2. step into a nether portal (you won't be able to go through)
3. ask an admin to tp you to the other dimension
4. interact (or have someone else interact) with a nether portal (the realm usually crashes after one interaction, but from our testing, it never needed more than 3)
I wasn't going to add this stuff to the bug report since it seems admin interaction is required to crash the server, but I've just found that if you are in a portal while the parrot is pathing to you in order to land on your shoulder, it will crash the realm as well. No admin interaction required.
People have mentioned that it happens randomly, but I don't believe that to be the case. What makes it seem random in my experience is that when it works, the chat/sign bug REALMS-9191 is happening... which seems to suggest that this issue only appears server-side.
I'd say it does seem to be related to the overly aggressive profanity filter which I assume they added more patterns to in a recent update, but the profanity filter changes words to asterisks even when the chaat bug occurs.. so either it filters on both sides, or it's not the chat filter