I'm willing to bet that Mono GC has improved a lot, but I don't think that Unity frequently updates the runtime. It's kind of painful, from what I've been told.
We are collaborating with Unity to assist on upgrading not only their runtime but also to adopt the latest and greatest GC.
Additionally in C# 7, 7.1 and the upcoming 7.x series there are many improvements that reduce object allocation (like ref structs, value type tasks and others)
Unity is just now updating the runtime for the first time in seven years. I think only the 2017.x releases have the new Mono, and most platforms use their IL2CPP AoT compiler I believe
