I backed a game a while ago on Kickstarter with native support for Windows, MacOS and Linux. Several weeks ago they sent us a message saying they were ditching the MacOS version due the architecture change. I wonder how it'll affect the whole ecosystem. I think it'll end being like an iOS on steroids.
How hard would it be to release an ARM version? Also out of curiosity, what game was it?
Depends on the engine the game uses. Games built on top of frameworks like Unity, Unreal Engine and the like get platform support basically "for free" as long as the framework in question supports the target platform.
Any in-house developed engine might either depend on 3rd party support (e.g. libraries like SFML[1]) or need to be recompiled for the new platform. Depending on architectural differences, optimisation and asset loading (in case endianness[2] differs) needs to be altered accordingly.
The main reason for a decision to cancel a release would be time and effort vs expected sales. The Mac market will be fragmented into Intel and Apple Silicon from now on and every "Mac version" would really be two separate ports (complete with separate QA and optimisation).
An independent studio might just not have the resources and experience to tackle this and a port that doesn't sell can effectively be worse than not releasing on a platform.
edit: [1] https://www.sfml-dev.org [2] https://en.wikipedia.org/wiki/Endianness