So, according to this very database, these highways are tagged with "name=Old Highway 91." I've mentioned this multiple times before, but "Old Highway 91" sounds like something better suited for a ref tag, instead of a name tag. These highways were once part of U.S. Route 91, and there are many human-readable variations of that, such as "Old Highway 91," "Old US 91," "Old Route 91," and even "Old US Highway 91." All these human-induced tags can be simplified into a tag like `ref=US 91 Old` ("Old" is not quite the same as "Hist," like "US 66 Hist"). I would sort of count these as official designations, since they are signed as Old Highway 91.