Hey Paul, thanks for reaching out own this topic!

The main reason these tag combination exists is to educate local mappers to use the appropriate scale tag instead of legal access tags which were heavily abused locally.
While using bicycle/foot/motorcycle/horse=yes can be technically correct if the way is legal and accessible, it has no effect in Thailand since paths/tracks are by default legally accessible to all kind of vehicles.
Such scheme led people using `bicycle/foot/motorcycle/horse=no` values on difficult/impossible ways which has been a subjective and conflicting matter.

Also I believe some values is better than none because in Thailand many paths/tracks were added from unknown source (imagery/import) and sometimes do not exist, so having some info when we know someone use a path on mtb is better than none.

But I am with you that these undocumented tags may cause issues with software maintenance (StreetComplete?), so a few options are possible:
- Remove them but we loose some info on whether those ways were feasible with a certain vehicles (hence exists)
- Convert them back to legal access tags  (motor_vehicle/motorcycle/foot/horse=yes) and find another way to educate mappers
- Move them to a prefix tag like `fixme:smoothness=unknown` or similar.

Your thoughts ?

Best,
Julien