Even though it's private at 3.7, extension authors can start using it at their risk if we decide METH_FASTCALL is public in 3.8 without any change from 3.7.
People can still wait for 3.8. Waiting 1.5 years for a feature is nothing when the software you're talking about is already 28 years. :) It's simply not worth the risk.
Of course. My idea is providing information to "early adaptors" who writes C extension manually.
PEP 580 is trying to expand METH_FASTCALL to custom function types in 3rd party library written in tools like Cython. But METH_FASTCALL cannot be used widely even for normal function types in 3rd party library yet.
Without publicating METH_FASTCALL, PEP 580 is useful only for libraries using private APIs. That's unhealthy.
So I think we should discuss about METH_FASTCALL publication before evaluating PEP 580. That's my main point, and "from 3.7" part is just a bonus, sorry.