@ATC Watcher/
When I was flying in Nepal all the aircraft in Nepal that flew into the mountains had deicing boots removed. I never seen a twin otter in Nepal with deicing boots. Even the ones yeti airlines(tara air) had were not working deicing boots. Most of the time they just had black paint on the leading edge or the boots would remain and they would remove the mechanism to increase payload and same for Dorniers 228.
About the routes some of the aircraft have on board GPS but they are not certified for IFR and they are no IFR routes in those regions. In the mountains it is strictly VFR only. The only thing you can do is do IFR approach back into your base airport which would be Kathmandu/Pokhara/Nepalgunj etc.
But no routes takes you to the mountains on IFR. But again the economics and as said lack of safety, the pilots do fly into IMC conditions without proper routes or procedures. Has been the norm there for long time.
GPS are usually handheld on most of the aircrafts but single engines are all equipped with garmin 430 and 530 with waas and some have weather radar too.
But it is just the culture there. Hopefully nepal learns something soon out of all the crashes and brings in some major changes!