Q: iPhone stuck with old email i shut down

Sign out of that Apple ID.  Your email account password and your Apple ID password are not connected.  While you may have made them the same thing, your email being decommissioned will not cause you to be unable to log out of the account on your iPhone.

Find My iPhone Activation Lock - Apple Support

If you forgot your Apple ID password - Apple Support

Contact Apple for help with Apple ID account security - Apple Support

You can change the email address that is associated with your Apple ID.

Change your Apple ID - Apple Support