This happens already. A few podcasts I support via Patreon have ad-free feeds that they supply to patrons as unique URLs for the RSS feeds. Presumably they can cut you off if they find that you then rebroadcast or share those URLs broadly.
The Economist also does this - if you have a digital subscription you get a private/unique URL with an RSS feed serving the complete audio edition. If you cancel the subscription the feed stops working.
People could produce ad-free versions of their podcast for Patrons.
Apple or other podcast vendors would also be in a good position to provide an open standard for subscription podcasts without ads.