Unity 5 came out in 2017 and it's not supported on Catalina.[1]
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?
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...