Hey there - the Steam account could only have been added by someone who had access to at least one of your logins (not your visible profile names) plus the corresponding password. So if you do not know who added it at all, you'd need to be securing your other accounts with a password change and 2FA, or else your accounts will never be safe. Unfortunately as of March 2025, it is not possible to unlink platform accounts at all. Before then, it was not possible to unlink accounts without authenticating them, so the change didn't affect you that much. Bungie will not remove the account on their end, and there are no workarounds nor exceptions being given. If you wish to play the game on Steam, you do have an Epic Games account linked too, and also as XBox Live is linked too, could play through the XBox for Windows PC store version.