Most indie games in the past 5+ years are made with a few middleware engines like unity or gamemaker and AFAIK it's a rebuild option to release a 64bit version.
Even just recompiling the game might be a challenge for a lot of developers in the "long tail" of the Mac game library.
> the notion that every developer of every app is part of the constant, incessant update loop that Apple encourages is fundamental to the problem. Someone's super personal narrative unity game from 2013 is not getting updated!
[1] https://twitter.com/MammonMachine/status/1181327259057082368
So devs of two year old games will need an old macOS install that can export their game to 64-bit, but also a new enough macOS install that can notarize their app (maybe these can both be Mojave -- but many indies don't own their own macs).
Also, Unity 5 is no longer available for purchase. If they're already using their license on another machine, they'll have to migrate it to these macOS installs.
Devs could upgrade to a more recent Unity and fix all the bugs, but to what benefit and at what cost?
[1]: https://forum.unity.com/threads/installing-unity-on-macos-ca...