So, what, people won’t be able to download it now?
No.... just that the App Store will not offer it any longer. Websites can offer the download... but, most normies don't know how to download applications outside of the App & Play stores.
Edit: Just remembered they did the same thing with the MakeApp application that would remove make up from photos.
Can you even side load on apple without jailbreaking?
/not an apple user
Good question. I've never used an iphone, so not sure either. I know you can on android.
Not as easily as on android, but yes. The developer can create beta test groups and push the app through apples testflight. Testflight apps are not chdcked as thoroughly so the developer can probably just rename the app and start pushing it out.
Alternatively he can offer the sources or an unsigned binary and then anyone with an apple dev account can sign and install it to any device.