Actually, the Apple ID still exists because Apple IDs can't be deleted (according to what I've heard from other people). Try creating a new Apple ID to sign in with if you haven't already.
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I have the same problem. I can sign in with my Apple ID, but I still get this message.
I do have a new one, but the phone keeps asking to validate the old one. It was indeed deleted. When trying to change the password I get an “Apple ID does not exist” message.
My iPhone keeps asking me to update the ID setting of an old Apple ID that no longer exists.