Indeed, the first example that comes to mind for me is highway=path. This tagging scheme uses the foot, bicycle, and horse access keys to indicate what the path is designed for, with the original intention of consolidating highway=footway, highway=cycleway, and highway=bridleway.
I agree that the idea of limiting the access keys to legal restrictions is a bit aspirational. It might still make sense as a rule of thumb, for situations in which legal restrictions would be apparent. However well-intentioned the focus on legal restrictions, applying that focus to many of these miscellaneous situations would result in hypotheticals that aren’t as relevant to a mapping project.
I suppose we have some more entries to add to the wiki’s lists of skunked tags and homonymous keys…