> it was well known that Unity was hindered by sticking with an old and out-of-date Mono, and they were very successful at deflecting the blame

So much this. According to a 2023 blog article from Unity [0], Unity uses Boehm GC. But Mono itself introduced another, generational GC called SGen [1] more than 10 years ago that became the default at some point. It is just Unity stuck on old mono versions, missing out on all the changes and improvements that went into Mono after their fork, essentially.

[0]: https://unity.com/blog/engine-platform/porting-unity-to-core... [1]: https://www.mono-project.com/docs/advanced/garbage-collector...