An IDFA also collected via any other service / broker provides such a feedback loop. Location data is reasonably valuable. Mapbox also offers a navigation API; this would be behavioral data from which companies like to try to derive interest and intent.
And has been mentioned a few times in the thread - even if one app author goes out of their way to reimplement the API in a privacy-preserving way, all it takes is one that doesn't to reasonably link all the other requests based on other data points. (IP would be more than sufficient - but since it's a location service, even if all the apps run the requests through a proxy, location-at-time and location/destination pairs are also useful to help link.)
In the end I don't think there's anything privacy-conscious app developers can do other than eschew external services as much as possible, clearly inform users when external services are used, and push for stronger regulation (from phone / OS manufacturers and the government) about data privacy.
This is the crux of it here. In particular, we seriously need regulations making the things Apple is pushing required by law, not something a company with its own vested interests pushes.
Particularly since as many point out whenever this is discussed, there is a bit of opacity with regards to Apple's own apps.