I'm trying to understand how the wp_users.deleted column is used by core.
It looks like it only exists for multisite installs, but wpmu_delete_user()
doesn't use it (it will delete the row and associated metadata).
get_user_by() doesn't take it into account, either. If I turn that flag on
for a user, their row gets highlighted in the Network > Users table, but I
don't see a way to get that state without updating the DB by hand.