How do I remove a defunct email address from Apple ID?

Do you mean that you want to change your primary Apple ID email account to a different one? If so:

Change your Apple ID - Apple Support

If that is what you are wanting to do, then BEFORE you do that, follow these sign out steps to make sure everything is signed out before you change the ID:

CHANGE APPLE ID - SIGN OUT STEPS

If you mean the email is in another place on your Apple account, please explain what, exactly you are trying to do.

Best,

GB

Followinghttps://support.apple.com/en-is/HT201356