> Just defining Py_LIMITED_API actually implies =0x03020000.

That's the intent, but if it were actually the case then this issue wouldn't exist :)

On 3.5.3, building with Py_LIMITED_API=1 will include APIs that should only be included when Py_LIMITED_API=0x03050000. You considered fixing that too likely to break existing users (and I agree), but that doesn't mean we shouldn't make it clear that it's not doing exactly the right thing.

> I have no idea where a #warn can be added.

pyport.h or pymacro.h are probably the best places. If you null-merge into 3.6 then we shouldn't have to worry about the warning showing up in later versions.