Can you list tags the preset would add to such single-use downhill-tracks?
I see them here (Austria) too. When panning the area with the German Map Style they get rendered as a white line with blue casing, regardless of cycleway or bicycle designated path. Everybody grown up here will read that signature as 2+m wide paved, like shown in the bike path picture on top. (mtb:scale will not be considered when rendering.)
I can imagine, highway=path;bicycle=yes;mtb=designated;mtb:scale=2+;etc. That should fix the rendering, but will not keep OSRM from routing there, see for issue – GraphHopper recently started to avoid these downhill races in routes for ordinary bikes. For good reason: Either the bike or the biker will break.
Did I mention, they intersect with the regular traffic network and sometimes provide shorter routes from one suburb to the other (in the city where I live.)