I like the idea of if, but the ephemerality of it is a biggest-killer there is for myself personaly. I think additional (or better as a default) version would be that all your posts are automatically hidden, unless chosen otherwise, and that the post retention policy at the time of posting of each reply be clearly indicated as well.
I cannot stand the apps and systems that, when blocking, just totally purge and puff the conversion out of existence, not just for the block-er, but the block-ed. Both sides have the right to their memories! Maybe Y poured their soul there, and they want to continue having access to those moments; X does not get to have ultimate control over that.
If X or Y want to block, then the blocking action should just lock and prevent further replies. (And there's definitely way more nuance possible here that I will be getting into here.)
The "nuclear" version would be to hide the blocker's part of conversation of which I also extremely disagree: everyone needs to learn, experience consequences, and evervigilantly practice to stand behind one's words — but those should always be remarkated with he fact that the stances, feelings, opinions expressed then should be considered ephemeral in an ever-evolving state, and not etched in stone. (And even add a short revision note tag function: "This is no longer considered valid, more info here / ask me more".
And while I have presented this example here as a private conversation between two people, my thoughts on the group conversations are very much aligned to these.
However, I am mainly of the opinion that block as a function should he reserved for the gravest of offences, and that people should (continue to) learn to settle their differences of opinion peacefully and as amicably as possible. Blocking willy-nilly does the exact opposite of it, and just builds the ever-more-impenetrable echo chamber(s) of samethink.