If the Steam account that is linked to your Bungie profile has been deleted by Steam, then it is not possible to unlink the account from your profile. The only option you have at this point, is to unlink your active platform accounts from your current profile, and create a new Bungie profile for them. This would mean losing any redeemed codes, such as emblems from promos and such, as those are tied to the profile they were redeemed on. But it will allow you to link your current platform accounts, and utilize cross-save. You may need to create a dummy or throwaway account, for a platform you do not currently have linked, as you cannot unlink a platform account that is being used to login to your profile. So you could make a dummy Twitch or PSN account, link that to your current profile, and use it to access the profile for the unlinking of your Xbox account.