Its a difficult one this. I had the opposite issue recently where a mapping site I was using was insisting on using a dual carriageway as is was foot=Not Specified so I had to switch this road to foot=no to get sensible route suggestions.
Therefore, simply switching on trunk roads may give too many false positives. Would the solution therfore be to allow trunk roads but exclude foot=not specified? Would need to do the work suggested earlier in the thread to update OSM to be foot=yes but that feels the “right” solution to me.
As examples, both correctly tagged as Trunk in OSM
