But then I unmention it and it's unreflected. This seems to be wrong. Perhaps another record like "Task Y does not mention this task anymore" could be added, but it looks better to just remove the initial record. Or, optionally, it would be sense-full to provide a person who did the mention a possibility to do it manually -- thus some judgement could be applied.

Audit-ability is a core feature of Phabricator. We're very limited in what we allow people to delete.

It is not too much different from removing comments. And this people are allowed to remove. (BTW I wonder what happens if one removes a comment with a mention)

Comment deleting is to help with accidental information leaks or outright spam. It is not our expectation that users will delete information from Phabricator and warn against it.