Another modern example, back country and overlanding routes. There's a decent amount of work and danger that goes into it, and not enough public interest for the big dogs to warrant mapping out the paths-less-traveled.

I get GPX routes and roll maps from TAT and BDR because these trails are not even on OpenStreetMaps.

Yes, there's some old stuff archived out there for free, but it's very limited and hit or miss. People who have collected a decent digital archive of things you would have found in a library decades ago deserve to make some money for their service.

Worth noting that OpenHistoricalMap does exist: https://www.openhistoricalmap.org/

Entirely plausible and even probable, though, that a commercial site has more historical map data than OHM does.