Unfortunately, we do not offer private subscription based feeds through hosting with ART19. We recommend a great tool called Memberful for this purpose: https://memberful.com/.
However, we do offer privatized RSS feeds but you still need to submit your private RSS Feed to a 3rd party app (i.e. Apple Subscriptions, Memberful, etc) for it to be available via whichever 3rd party app you decide to use.