To answer my own question: a non-renewing subscription In-App purchase type is acceptable for static features in apps.

Unfortunately Apple's App Review process is usually done by staff in a hurry which means they might miss things like the fact that you already have versions live on the App Store with these features. They might just look at the app you've submitted and not compare it to what's already live on the App Store. Right now I doubt there's much to do about this unfortunately. I'm just curious: do you just explain to Apple in the chat or do you have to go trough the process of filling out the appeal form, and how long does it usually take for them to approve your app after you've submitted your appeal?