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)

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.

Hmm, well if those T4267's alias accounts would support non-ASCII then yes for this case. But I am unable to see why not to complete this one :)

I am not aware. Shall I create one?

Because we live in Unicode era and there are languages which use not just ASCII letters? Чесно!