Maybe mtb:scale=ridable would be a better value? We should describe the intention of the value on the wiki, but preferably choose a value that is as self-explanatory as possible.
A single GPS track on Strava should not be enough to map it as an mtb trail, but several of them could be, especially if also supported by an account.
mtb:scale=impassable would mean “impassable for the average mountainbiker”, while mtb:scale=6 means “impassable for everybody”