It's not restricted to software features, though. Things like ebooks and (???) event tickets also have a 30% cut (Facebook had an angry dispute over this).
This is why the Kindle app doesn’t let you buy anything on iOS. Such an awkward flow because you can read a free sample of a book and as soon as you finish and are ready to continue it just appears that there’s no way to buy it.
The behavior is the same on Android.
Bandcamp's iOS app doesn't have purchases, presumably because of this.
lol I just saw a recent Bandcamp update for iOS where the changelog said it "updated payment workflow" or something and it got me real confused because the app has never let you buy anything