If your question is specific to the Ars Technica subscription, there is no authentication. Each subscriber is given a unique feed link (for whatever they want to subscribe to) so any fetch operation for the link is tied to the subscriber. Ars says that it monitors the use of feeds and will take action if any abuse is detected, but doesn’t go too deep into explaining that part.
Some other services like Stratechery follow the same model, AFAIK.