This was a problem on estate tracks in Scotland, that are, or were, frequently tagged as footpaths, with no access permission for cycles, but it seems that there's been either a bulk change of the tags, or that cycle.travel is now allowing routing on all Scottish paths.
[edit] It's retagging: c.t still won't use the track down the side of Loch Garry.
Interestingly, the unrideable sections are showing up as cycleways

Example: Glen Brown, between Dorback Lodge and Tomintoul, on the cycle.travel recommended route.

You follow the valley bottom, crossing the stream multiple times, then turn right and thrash up the hillside just after the trees end.
It's been used by a land rover occasionally, but not consistently enough that these's much of a defined track.