IME: less than you’d think. I know one major C# project that’s only half completed its migration and that huge (both in terms of what’s been achieved and what still needs doing). There’s another that keeps it for the front end because it runs on >10,000 client machines. All the others, big or small, have migrated with small carve-outs for stuff .NET Core doesn’t and probably never will support.

From recent experience I'd put it slightly higher on the old framework, but plenty of new dev on v5+ and I think sticking on framework is worse at this point.

Though I would say that breaking changes since Core 3 have been pretty limited. V5 unified .net under the new core as the path forward for framework users as well.