Thanks for your feedback.

The source is mostly high resolution LIDAR DEM data.

There is no real consensus as far I can see for these old trenches : https://overpass-turbo.eu/s/2duQ

Adding "historic=" or "disused=" is an option. https://taginfo.openstreetmap.org/tags/military=trench#combinations

"disused:military=trench" may be more appropriate, but is not much used.