I left a lot of open notes scattered across MO over the years, yikes. This reads like a todo for myself that I left behind a while ago; sorry I never got around to it.

As far as valid, I've found discrepancies before between MoDOT's own travelway data and Bing StreetSide for supplemental state route designations.

If by valid you refer to usage rights, the road network data used by MoDOT's Traveler Map is the same stuff offered through MSDIS which is cleared for use:

https://data-msdis.opendata.arcgis.com/datasets/MSDIS::mo-modot-roads-routes/about

https://wiki.openstreetmap.org/wiki/Potential_datasources/Local_data#Missouri_Spatial_Data_Information_Service

Check out MoDOT's HPMAPs as well. It presents the same open data but in a less distracting way imho: https://traveler.modot.org/hpmaps/

Maybe aerial imagery can locate the transition line from state to municipal maintenance, which could be visual evidence for the true terminus of DD.