Bungie cannot unlink accounts from user's profile for security reasons. As you have deleted a Steam account, it is an issue with Steam support, not Bungie. Also, you will need to deactivate cross-save in order to remove the wrong Steam. This will incur a 90-day security lockout during which you cannot reactivate cross-save.

Unfortunately there is nothing Bungie can do in this case. If Steam are saying they can't restore a deleted account then your cross-save cannot be unbound.

You can try and see if Bungie would be able to deactivate the cross-save for you by contacting them using the form at the bottom of [url=https://www.bungie.net/en/Help/Troubleshoot?oid=48099]this article[/url]. Note that, if they do deactivate cross-save for you, you will need to migrate your active accounts to a new Bungie profile as the deleted account cannot be removed from the profile.