Once you link a Microsoft account on your Playstation if you forget the email address that is associated with that account you can never receive any support from Microsoft, Minecraft, or Playstation.  Since the unknown linked account cannot be signed into the Realms server that this user is a part of cannot log in and play, the only resource seems to be to create a new Playstation account and then purchase Minecraft again to be able to link to another Microsoft account.

Somewhere, one of the three players here have to know what email address is associated with the account name that is shown on the screen, but none of the three players will help.