I've searched and found that the license for the map is CC BY-NC-SA 2.5 BG (https://bgmountains.org/en/terms-of-use) and I think it should be ok to use it. Also in this forum https://community.openstreetmap.org/t/topic/76303 one of the authors said that they have plans to add the data to OSM.

Hi,
It comes from https://map.bgmountains.org/#map=8/2810000/5250000/0
https://old.bgmountains.org/en/bgmountainsgpsmap/223-licence
It is amazing map for hiking in Bulgaria. However, I'm not sure if the license is OSM compatible.