We spent a lot of time on this when I was at Segment. It's incredibly hard for an evented system to go back through archives and delete every data from a user,

We spent a lot of time on this when I was at Segment. It's incredibly hard for an evented system to go back through archives and delete every data from a user, but it's possible. It should be much easier for Twitter to do this, so there are no user merges (anonymous user becomes identified and merges), but I'm not sure about their data storage. This should have been done years ago.