Hey everyone,
I am currently developing an iPhone app for my best friend who is a creator on Patreon (she would like to stay anonymous).
We currently are facing an issue regarding the login via Patreon. We have a website running under patreon.exclusive.example. It has a Login button which redirects you to Patreon and you perform the login itself, after that the server side checks if you are an active patron and if yes you get access. Otherwise you get redirected to her Patreon page.
This is a flow chart of that process:
We already have a working Android App which follows a similar flow. You tap Login in the app and in the default browser https://patreon.exclusive.example/login?mobile=true opens. After that you perform the patreon login, the server side performs the active patron check and after that redirects you to example.exclusive.patreon.
This is, again, a flow chart of that process:
On iOS this flow does not work and gets rejected by Apples policy. You are not allowed to open a page allowing payments outside of your app. So a friend of mine and I developed a different flow for iOS. For this again a flow chart, it is a bit clearer than text 
This flow works if we get a specific entitlement approved by Apple, called External Link Account Entitlement. If we don’t get that entitlement this flow won’t work either.
The Patron exclusive website does not have its own user database and fully relies on Patreon for user management. That gives us the issue, that we cannot use the system login feature which would allow to choose Patreon. We cannot use that because Login with Apple is a requirement which simply doesn’t work in our case.
Another option, the Apple prefered way, would be in App purchases, which also don’t make sense, since you would pay the same price with less benefits and only access via your iPhone, not the web.
Has anyone ever build an iOS app that has a login via Patreon and no other mechanism of accessing exclusive content?
Thank everybody who helps and have a great weekend
Imanuel