Thanks for your detailed explanations. I am riding mtb since more than 30 years and also had a good time on dirt bikes, most of the time on tracks, trails and pathless ground in Africa. The issue here is not to distinguish dirt bike trails made for the purpose from mtb trails, but to tag normal pathways through the bush and in such cases one can normally say if the path is usable by foot and by dirt bike, it can also be used by mtb, even if one might have to carry the bike on the shoulder for a very bad section.
Again it is simple enough to go for practicability=dirt_bike;mtb;foot or the like to avoid any misinterpretation.