If you lose your Matrix account bound to a 3PID, there appears to be no way you can erase it (also: phone numbers change owners)

[reivilibre]

If you register a 3PID that's in use, you get a THREEPID_IN_USE error.
That's all well and good until you don't have the ability to unbind it because either:

• it's a recycled phone number
• you lost your Matrix account that was originally bound to it

This is bad from a usability perspective and also somewhat data protection (I guess people will have to contact support to get it removed if they can't do it themselves, for now).

(This came up because someone lost their old account and couldn't remove the 3PID, so it's not just an academic problem.)

[clokep]

Is this an implementation issue (e.g. with sydent) or a spec problem or somewhere in-between?