Are you allowed to promote a Patreon page on your iOS app?
Let’s assume you’re not a charity or fundraising for one.


Technically, no since in 3.2.1 (vii) it says you’re allowed to promote voluntary giving, provided 100% of the funds go to the recipient — but Patreon takes a cut of the funds pledged / gifted. 
Realistically, probably yes. If your app is otherwise fine, Apple might not reject you solely for having to pay processing fees. 


As long as you’re not just trying to dodge in-app purchase, I would say make your app and submit it for review. Also, look at IAP for a tip jar I addition to the patreon option. People can and will tip you if they love the app. Also, worst case is you have to find a payment processor where you pay all transaction fees up front or separately so Apple can see 100% of the linked gift goes to you.

Also, if this is all your app does, don’t bother. You’ll need a compelling app that provides obvious value for free or paid and can’t just be a “here’s my patreon, fund me” shell.