> Is there a history of that here?

I have been maintaining not-super-successful open source projects, and I've had to deal with entitled jerks. Every. Single. Time. I am totally convinced that any successful open source project sees a lot more of that.

> Were there earlier clear statements of expectations (like CONTRIBUTING.md) that expressed the same expectations, but in a straightforward way, that people just willfully disregarded?

IMO it's not needed. I don't have to clearly state expectations: I open source my code, you're entitled to exactly what the licence says. The CONTRIBUTING.md is more some kind of documentation, trying to avoid having to repeat the same thing for each contribution. But I don't think anyone would write "we commit to providing free support and free work someone asks for it" in there :-).