The OP here is just asking for donations; users don't get anything in return.
The App Store Review Guidelines only say "If you want to unlock features or functionality within your app [...] you must use in-app purchase."
I haven't tried it (yet), but it looks like you might be ok with a Patreon link from an iOS app. The review process can be capricious, of course.
> Apps in this section cannot, either within the app or through communications sent to points of contact obtained from account registration within the app (like email or text), encourage users to use a purchasing method other than in-app purchase.
> (vii) Apps may enable individual users to give a monetary gift to another individual without using in-app purchase, provided that (a) the gift is a completely optional choice by the giver, and (b) 100% of the funds go to the receiver of the gift. However, a gift that is connected to or associated at any point in time with receiving digital content or services must use in-app purchase.