I just found an old discussion about the iOS client in ownCloud which was closed-source in the beginning as well. Then the sources were published but still no own version could be distributed:
In this discussion I found a link to a real open source iOS client that can be freely modified and distributed (under MIT license):
Probably it can’t compete with the current ownCloud-iOS-app and closed-source Nextcloud-iOS-app (yet), but it’s a start. It is supposed to work with oC 8.1. Could this be the start for a proper Nextcloud iOS app?
AFAIK you cant publish Open Source apps on the app store because Apple requires you to sign a contract that requires you to not publish the app on any other store. This however is incompatible with most? licenses
I think so, however non app store apps on iOS are pretty hard, especially because you need a bunch of signed certificates to access additional APIs, so while you probably could compile the app it could be worthless because you cant access things like network on iOS.