The main potential benefit I see to keeping the special variable declaration is that it may help avoid a certain category of error: calling _Py_ONCE_VAR_INIT on a stack local pointer reference (which would leave the global array with a reference to nonsense). While we don't care if the once_vars are static or not, we do care that they're not allocated on the call stack, as otherwise they won't be around for Py_Finalize() to clean up.

On the other hand, _Py_SET_ONCE is nice and easy to explain "it's similar to _Py_SETREF, but: 1) doesn't do anything if the reference is already set; and 2) registers the reference to be cleaned up in Py_Finalize"

Also interesting is the fact that you can still use _Py_SETREF to change a reference that was initialized with _Py_SET_ONCE without breaking anything. From that point of view, a better name might be _Py_SET_FINALIZED, emphasising the fact that it registers the pointer for finalization over the fact that it's a no-op when run on an already set pointer.