Hi,

why to keep it in osm ?
With no type, this relation is useless, no  ?
If this route is now impossible for mtb (for example because of a big landslide), to delete the relation is maybe better.
But if people can use it, the tags type/route could be restored and the tag ref could be changed into old_ref (with a description or a note to explain why now an old_ref).
Even if the route is no more an official route, maybe it is ok to keep it in osm ?

Best regards 