As first spotted by Steve Troughton-Smith, release notes for the latest beta build of Xcode include a major development: Mac apps can soon be included as universal purchases with their iPhone and iPad companions.
I think this has been a long time coming, and we’ll probably start to see even more unification of the App Stores soon.
From a personal perspective this is also quite interesting. As I have a universal (iOS and iPadOS) version of Text Case, and also a separate macOS version. So right now you need to pay separately for each version. Selling them as one entirely universal app would either mean giving one away “for free”, or increasing the price. Alternatively, a bundle may be the better easy to go.