There should be some discretion on the part of the maintainer. Maryland chooses to number some of its abandoned alignments and frontages with the same numbering system as its normal state highways. Virginia does this with frontages and stubs designed to preserve property access that was taken away by a primary route's reconstruction. Virginia considers these routes primary routes and posts 3/4 of them, but they have a separate numbering convention (F-routes) from their state route numbering. So Virginia's F-routes can be easily ignored in TM until someone were to decide the whole system should be included.
But Maryland definitely has unposted routes that are real roads, often of independent utility, sometimes on maps, and are subject to posting at any time, like the recent MD 219 and MD 368/MD 568 which went decades without posting then suddenly were fully posted one day.
And of course we should be including state routes that are posted but somehow deemed insufficiently posted (NJ and VA definitely have these).
