First of all, I understand that British mappers no longer strictly apply highway=unclassified to what the authorities call “unclassified roads” and occasionally mark with an erroneous “U” road number sign. It’s technically a misnomer along with highway=trunk (not necessarily the designated trunk roads) and highway=primary (not necessarily the green primary routes). Instead, the documented definitions basically distinguish highway=unclassified from highway=residential solely based on abutters=*.
Across the pond, the U.S. mapping community has grappled for years with highway=unclassified. At first, when there was a strong desire to map highway=* values one-for-one to route networks, mappers in some states established a baseline applying unclassified to the roads maintained by admin_level=7 governments, while mappers in other states avoided the tag altogether because there is no government at that level.
For example, in the Midwest, tertiary was generally used for county roads, unclassified for township roads, and residential for municipal roads, regardless of abutters. Depending on the area, township roads could be well-constructed urban roads, rural dirt paths, or anything in between. For completeness, here are the exemplar images in a visual field guide for Ohio largely written during this era:
It was a very regional approach, but we saw the classification tags as nothing more than a scale to fit our own local needs into. These days, we’ve grown out of this heuristic. For consistency with the rest of the world, nowadays, we generally apply highway=unclassified to local streets without residential abutters. But it sure feels like an arbitrary distinction better captured as abutters=* or landuse areas. For this reason, there have been occasional musings about abolishing highway=unclassified to simplify the classification system.