Was in Altay about a month ago. No mosquitoes in the mountains. The only place which had some mosquitoes was Kedrachi, bexause horses go through here. Would highly recommend trekking to Belukha mountain.
https://www.google.com/maps/d/u/0/viewer?mid=10m-Y7pFmGG68drn2PdFrAtHCu4XyLrng&ll=50.028110355261646%2C86.43992290000006&z=11