I’d like to point out that bicycle=yes/no is very often de-facto is used for passability, (even though the official guidelines seem to be about legality).
In Israel Hiking map, a popular OSM based app in Israel, purple indicates “passable but difficulty unknown”. a highway=path is brown, but becomes purple by virtue of having bicycle=yes, adding mtb:scale augments the color and changes it according to difficulty.
You have to switch to the MTB basemap to see this behavior.
So, according to that logic, a rideable path with unknown difficulty is simply highway=path, bicycle=yes.