I recently came across this website: https://www.rowmaps.com/osmand/ that provides rights of way (footpath, bridleway and byway) data that can be overlaid on maps in Locus. The page I've given gives details on how to use it with Osmand but it works similarly with Locus.
The data comes from local authorities open database and is more comprehensive than the available Mapsforge-based maps, though the mapping is less precise.
The only drawback is that there is so much data that map drawing can be very slow unless you restrict what is displayed to the nearest 50 tracks for example.
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