Only the mapper having tagged these will know. These are individually created values not really in line with the description on the wiki page.
I’d say using the key practicability=* by including what a track may be usable for is more simple than excluding what a track may not be usable for by using the key practicability:*=*.
Let’s say there is a single track (path) which according to the mappers estimation could be usable by a skilled mtb or dirt bike rider. A track usable for a dirt bike can be assumed to be useable by mtb or foot as well, so one could simply tag it as
practicability=dirt_bike
If you don’t trust everybody will understand that “dirt_bike” includes “mtb” and “foot” you would go for
practicability=dirt_bike;mtb;foot
How would you get the same result by excluding values?
practicability:motorcycle=no would also exclude dirt bikes which are also motorcycles of a special kind. So you would need another key to make clear that “normal” motorcycles are excluded but dirt bikes could be used.
Besides that replacing the key practicabilit=* with 17K uses now by a new key practical=* or practical*=* for the sake of brevity would not make much sense imo.
Anyhow I still believe the best way to specify any track or path is by using the existing tags for surface, smoothness, width and sac/mtb scale while practicability could just be a compromise for those who do not want to get into the details.