In the case of the fossgiss foot routing profile, I’d then call that a greylist instead. It might make sense there, if this is exemplary code, then the enumeration is just in place to give pointers to the base canonical tagging to ease refining. Of course, the logic employed fails on the wrong side with erroneous tagging like e.g. difficult_mountain_hiking. Should be solvable in code though. Who is to file the issue?
Of the routers tested so far, only BRouter seems to route anything in the highway=* key. Most others select for complete tags, like highway=path, unless there is e.g. a hiking relation on top of it. Martinswand is a true via_ferrata. In my eyes, routing there will not bring anybody into doing something stupid, it will only make people angry at the router, and that starting on the approach paths 
Who is to file the issue?
I remember you calling sac_scale a local thing. I wholeheartedly agree with you on this. Still I ask: Can you come up with a universal scheme?