threatening emails from Norton telling me that it has expired

It is not from Norton, it is from a scammer wanting you to go through their service to purchase Norton.

Although it is not needed, Norton is a reputable company and doesn't send out such spam emails telling you your subscription has expired.

Read more about it here -

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support 

Ignore and delete any scam messages like this.