Why did Apple remove Red Reign?

While I can't give you an answer I can remind you that games in Arcade are not written by Apple, Apple does not write games. It is entirely possible that the developer/owner of Red Reign pulled it from the app store for their own reasons. It is also possible that Apple found code that violates their security guidelines and took it off until the developer fixes it to their satisfaction. There are numerous reasons why the game may have disappeared.

You should contact the developer of Red Reign and see if they can answer your question.

Yes, this is my favorite game. Oddly enough, I have an old iPhone 7 that was off when Red Reign was removed. I charged the old phone, and Red Reign still works on it. So, I quickly turned off wifi on this phone so that it wouldn't update. Although I miss playing it on my iPad, I'll take the small iPhone screen for my Red Reign fix.

Don't know, I'm pretty shocked as I play that game pretty often