Correct, I don’t mean to replace the definition of legal access tags. Some mappers in my area use bicycle=yes/no to classify the difficulty of a path, probably out of laziness or unfamiliarity with scale tags. These paths have no legal restrictions. For their purpose, bicycle=no/yes could mean any scale, and will make routers avoid paths that more advanced riders could use, so I want to change this data and raise awareness about the scale tags.

I understand your concerns about mtb:scale but this is the main tag supported by outdoor applications, so I want to refer them to this tag first. The more people start using mtb:scale, the more chance people will want to look into other subtags.

I have the feeling the dirtbike scale definitions will need some iteration and time to refine since there exist no official standard difficulty scales for dirtbikes.

Would you suggest then creating a country-specific page first to try it out?
Once we have a solid scale system, we could make it available globally.