Maybe there’s a misunderstanding here. If a reliable source provides GPS data and says a trail is rideable, that’s not a subjective guess. Mapping without a ground survey is common—people map roads and buildings from imagery all the time.
That said, I fully agree that assuming a trail is MTB-suitable without local knowledge is subjective, error-prone, and shouldn’t go into OSM.
Just to clarify: this poll is about reliable local knowledge of trail rideability. Many areas are poorly mapped, with few active mappers but many users—some of whom are happy to share useful info even if they don’t map themselves.