How can I delete an old and unused Apple Account?

For information about how to delete an Apple Account (formerly “Apple ID”), read this support article —> How to delete your Apple Account - Apple Support

Carefully and thoroughly read that article for information about what happens when you delete your Apple Account. For example:

"Make sure that you sign out of all your devices to avoid issues with apps that use your Apple Account or iCloud. After your account is deleted, you can’t sign out of iCloud or turn off Find My iPhone Activation Lock on your devices. If you forget to sign out, you might not be able to use your device when your account is deleted."

Login with those account credentials (the account you desire to delete) at: https://privacy.apple.com

Follow the link to delete the account.

Play close attention to all instructions and warnings provided as account deletion is irreversible.

Failure to follow all the instructions can even lock you out of your own devices … permanently.