What is the effect of altitude on true airspeed?
If you know GS and local wind speed, the TAS always the same, regardless of altitude. The true airspeed is called "true" because it is exactly how fast you are moving relative to the air. Picture it as a person in a weather balloon (which does not move relative to the surrounding air, i.e., TAS=0), pointing a radar gun at your aircraft to measure its speed. The results would be the same regardless of altitude, temperature, pressure, etc. This is what sets TAS apart from pretty much any other speed like IAS, CAS, EAS and Mach.

Only at truly ridiculous altitudes you would have some problems with calculating TAS, due to the increased orbital radius from the center of the Earth. However, at this point, you're well into the near vacuum of space which means that the whole concept of 'air' speed is quite silly.