I don't know if mountains have, and where, always impervious to cliff jumping. But considering there res files where not updated. ... If Cliff jumping was possible there, it still might be possible.
+ seems some mountain res files might be in need of a general cleanup, as some of the mountain res files have a ridiculous large NEG layer. (+8000 compared to a normal size of 17 for other ridge res files.

)
gfx\terobjs\ridges\mountain\ee.01 1 13879 [('image', 5735), ('neg', 8108)]
gfx\terobjs\ridges\mountain\es.01 1 12140 [('image', 3996), ('neg', 8108)]
gfx\terobjs\ridges\mountain\ne.01 1 13635 [('image', 5487), ('neg', 8112)]
gfx\terobjs\ridges\mountain\ns.01 1 13161 [('image', 5013), ('neg', 8112)]
gfx\terobjs\ridges\mountain\se.01 1 12143 [('image', 3991), ('neg', 8116)]
gfx\terobjs\ridges\mountain\ss.01 1 13943 [('image', 5795), ('neg', 8112)]
gfx\terobjs\ridges\mountain\we.01 1 13237 [('image', 5093), ('neg', 8108)]
gfx\terobjs\ridges\mountain\ws.01 1 13590 [('image', 5446), ('neg', 8108)]