timings doesn't seem especially hard to stabilize; I'll take a look at it.
Third party tools might rely on the output. This is okay for things read by humans (say the error messages) but fails for things that tools are parsing, like data needed for code coverage tools. Stabilization of instrument-coverage, as proposed, is a bad idea in my opinion. I'm sad that the concerns have not been addressed: https://github.com/rust-lang/rust/pull/90132#issuecomment-94...
It depends on whether what you're stabilizing is an output format or a set of functionality. I think it's reasonable to do the latter.
(If you'd like to talk about that further, I would suggest Zulip.)
As another poster down-thread said, human output can change at any time and this shouldn't be viewed as a stability problem. And if the machine output is still in the air then maybe it pays off to branch it out in a separate flag?