This one is unusual because it won't break old code being brought forward, only the other way around, and theoretically there are lots of things going the other direction that would break (though most of them are explicit, not silent).
Backward compatibility is the ability to run old code with new interpreters. This is not broken here.
What's broken is the ability to run new code on old interpreters. But this is already broken at every python update (new operators, methods, syntactic sugar..). We could call it reverse backward compatibility.
- backwards compatible code: new code can run on an old interpreter
- backwards compatible interpreter: old code can run on a new interpreter
EDIT: After some thought, you're right. The second description is the reasonable interpretation.
"python 3.7 is backwards compatible with python 3.6"
(Driving to an airport) "Okay, we gotta pick a road. Arrivals or departures? We're arriving, but then we're departing."