You have a point here: For shared-use paths in the first sense - not any path where cycling is legal, but ways “designed to accommodate the movement of pedestrians and cyclists” - there is already a well established combination of tags.
That combination of tags is foot=designated plus bicycle=designated… plus either highway=cycleway or highway=path, depending on the preference of mappers. (UK uses cycleway, Germany uses path, etc.)
Sorry, I don’t understand what you meant by “the usage”