You seem confused. There is no mysterious GPL only label in the kernel. The whole thing is licensed under the GPL v2 period. It’s just that some ABI were broken for an unrelated reason as Linux doesn’t guaranty ABI stability and OpenZFS can’t find an alternative which satisfy their dependency needs. The kernel team doesn’t care about out of tree code when making changes. It has always been the rule.
Yes, you are right. I didn’t know the API was tagged. My take seems indeed a bit too extreme.
The kernel does care about enforcing the GPL explicitly on some of the interface it presents to modules to ensure the openness of the code which is indeed a form of statement in favour of open code.
I don’t think the situation was the same regarding compiling only with GCC - after all clang is free software - and I think the heart of my argument still hold: the kernel community makes decisions mostly for reason related to the kernel - even there they just want to force code to be mainlined - rather than for the movement like the FSS.
