Measuring slope in percentage or permille is pretty common in railway especially if you're using metric, as it makes some of the maths easier. E.g. at 1 permille, you gain/lose 1 meter per kilometer. 50‰ = 50m per km.
I could see the Russians using it but I don't think I've ever heard of it.
Sounds more useful for flight path/glideslope than for AoA or actual deck angle, though.