If an user is removed, of course its repositories are removed and all the information contained too, what happens with the contributions to external repositories? Like commits in a organisation or into other users repositories.
Is any other activity history removed?
Commits in other repositories are not changed at all.
Commits are identified by author email address, as part of git itself.
If the author user is deleted, those commits will no longer link to an email address, but will not otherwise change.