In my understanding, this depends on what action Apple exactly takes. If it just removes it from the App Store, all existing installations on devices would remain and continue to work. If Apple revokes the developer's certificate, then the app would likely fail to launch sometime soon and would be removed.
I'm sure there are gaps in my understanding. Corrections are welcome.
I believe these are the different levels they have at their disposal:
- Remotely delete from everyone's devices (Apple has never used this capability, they say the ability is there in the case of malware)
- Remove from the App Store completely (keeps running if you had it installed, you can back it up to iTunes and reinstall from there, can't download from the store. I think they did this with apps that worked their way around AT&T tethering restrictions back in the day when that was a thing)
- Removed from sale (if you installed it before, you can still re-download it from your App Store purchase history)
I don't think certificate revocation is applicable to app store apps - e.g. I can revoke my own dev distribution certificate and it only kills self-hosted Ad Hoc builds. They may have per-developer app store certs only available internally as well but I think they'd use one of the capabilities above instead
I'm sure there are gaps in my understanding. Corrections are welcome.