Yes they stopped caring after C89, Plan 9 C compiler was a mix of C89 with extensions.
Still they are the authors, and withdrawing from WG14, going their own direction with Alef, Limbo, and finally Go, kind of proves the point they considered C done for its original purpose.
> kind of proves the point
I don't think it proves anything other than the wanted to work on something else.
It is a bit of presumptuous for one to assume to know better about something than the original authors.
Something was designed in a certain way, accomplished its goal of writing portable UNIX, and then design by committee took over.
The authors, with their experience moved on, the committee and the audience doubled down on the flaws.