I have an old PSN account that has since been deleted attached to my Bungie profile. I cannot activate cross-save for my Xbox-PC accounts because I need to authenticate my PSN as well. The problem arises when I need to sign-into the PSN account as it no longer exists. Upon going to my account settings to try to unlink my PSN from my Bungie profile, for some daft reason, you need to sign in to unlink it too.
Yes, to enable cross save all linked accounts must be authenticated and yes to unlink any account requires you to authenticate the account you are trying to unlink. This is to prevent unauthorized changes to a player's Bungie profile. You need to work with Sony to recover that PSN account. Bungie does not add or remove platform accounts to player's profiles. If you are unable to retrieve the PSN account's login credentials then you have two choices. 1. Leave it alone, but you won't be able to utilize cross save and you'd never be able to link another PSN account to this profile. or 2. Unlink all your good accounts from this profile, log out of bungie.net, click the join button to create a new profile and link your good accounts to it. Keep in mind choosing option #2 means your time on bungie.net will be reset to 0 and any emblem codes you redeemed on your old profile will not be available on the new profile.