Hey there. That Steam account could only have been added by someone who knows at least one of your logins, not just the visible profile name, but your actual login username, plus password, of one of your platform accounts. If you have not done so already, you should change your passwords and consider setting up two-factor authentication. If you're unable to find out how someone obtained your details, your accounts may never be safe. Bungie does not forcibly unlink accounts from a cross-save setup, meaning if it's an account you are unable to authenticate, for any reason, it is unfortunately stuck. Cross-save can only be cancelled by the user, and this is a mandatory step before accounts can be unlinked. You mentioned Steam and account termination. If an account in a cross-save setup is deleted by the platform, then it can no longer be authenticated and will forever be stuck in your cross-save, account deletion is not a solution.