What happens if you no longer have an old phone number on your phone and forgot to deregister a phone number you don’t have anymore

how do you deregister a phone number you no longer use if you’re unable to locate it due to one of your emails being compromised and then deleted the email but it seems like I missed something when I tried to update my account and an old email phone number is still somewhere in my identity but I have no access to deregister them. I think someone else is using them because it appears that old device that was reset wasn’t reset properly so it’s causing problems. What should I do to fix this mess. I know it’s confusing but it’s like another rogue email phone number are connected to my email address I use for Apple ID but can’t find it?? To deregister should I just delete my email deactivate and start new? How do I save my purchases when doing this?