Since we don’t have general tags for usability by transport mode, there are some highway-types that limit access by usablity. highway=steps for example. You are not forbidden to drive down steps with your bike (or motorbike), yet highway=steps implies access=no + foot=yes. Do we want to route bicyclists via steps or even ladders? Probably not. Do you tag every highway=steps with additional horse=yes + bicycle=yes + motorcycle=yes + moped=yes? I’m sure you don’t.
Lawmakers in general like you to apply some common sense. So just because there is no sign saying “bicycles forbidden” on a scramble, it doesn’t mean a judge wouldn’t rule it as a “schnapsidee” (crackpot idea) to ride there.
We haven’t agreed on any new tags yet, we’re pretty much in the spitballing phase of things, and we cretainly don’t want to end up with 50% overlap of tags. I do understand that some suggestions might make people rip off their hair, but at this stage of things, nothing should be forbidden to suggest. Except pictures of shoes.
About singletrack and trails: All singletrack are trails, but not all trails are singletrack. A singletrack is just trail that’s so narrow that when 2 people meet, one has to leave the trail to let the other person pass. There are singletrail that hikers, equestrians and (mountain)bikers all use at the same time, but, and that’s the one we’re talking about, there are also singletrack that are not open to pedestrians or equestrians, or at least limited as “use at your own risk”. Of course, this will vary by country. I’m not an avid mountainbiker, but I’m assuming that the singletrack suggestion was referring only to those exclusive to bikers.
If some types would overlap, it might make sense to resolve these via multiple values or dedicated tags like singletrack=yes. Of course, you can also add a width to a path, but it’s generally easier to estimate singletrack vs. doubletrack than 1.5m vs. 1.3m.