It’s odd how Apple picks and chooses where to enforce this requirement. Netflix has been using their own billing system for years, yet I can still download Netflix on iOS, tvOS app stores. Makes me wonder if there is an under the table deal between Netflix and Apple. On the topic of Patreon vs Apple. I am not a lawyer but this seems to have the same basis as the Epic Games vs Apple litigation.
AFAIK this is because you can not make changes to your subscription within the app. You have to visit the Netflix homepage. There is no “pay” button in Netflix, so they do not have to use apple’s payment system…
iOS and Google apps just used to view content.
Patreon is a relatively small company so velocity to complete this effort even amongst multiple teams should be at most a few weeks. If this was a Fortune 500, I would probably estimate it at 2-3 months.