Why the ceiling moves
A liquid boils at the temperature where its vapour pressure equals the pressure of the atmosphere pressing on it. Lower the atmospheric pressure and the liquid reaches that point sooner, so it boils cooler. Air pressure falls with altitude, so water boils progressively below one hundred degrees as you go up. Crucially, once water is boiling it cannot get any hotter in an open vessel — additional heat converts liquid to steam rather than raising the temperature. A kettle at altitude therefore does not deliver merely slightly cooler water; it delivers a hard ceiling you cannot exceed.
Roughly how much
As an approximation, boiling point falls by about one degree for every three hundred metres of elevation, though the exact figure varies with weather as well as height. A city at fifteen hundred metres boils water somewhere in the mid-nineties; at two and a half thousand metres it is closer to the low nineties; on the Tibetan plateau it can be under ninety. Those are approximations and are given as such — a barometer would tell you more than an elevation figure does, since a deep low-pressure system moves the number too, just by much less.
Which teas notice
The teas that want the top of the range are the ones that suffer: heavily roasted oolong, compressed dark tea, robust black tea and anything traditionally boiled. These are already asking for as much heat as an open vessel provides, and losing several degrees off the maximum shows up as a thinner, less opened cup. It is a plausible part of why traditions at high altitude lean so heavily on prolonged boiling rather than steeping — boiling for a long time at a lower temperature is one of the few routes available to extracting dense compressed leaf when the ceiling has dropped.
What to adjust, since temperature is not available
You cannot raise the ceiling, so the levers are the other two. More leaf puts more dissolved material into the same volume at any given extraction level. More time moves further along the extraction curve. For compressed and roasted teas at altitude, extending the steep and increasing the dose together is the sensible compensation, and simmering rather than steeping is a legitimate answer for dark tea specifically. What does not work is repeatedly re-boiling in the hope of getting hotter water, which achieves nothing except driving off dissolved gases and flattening the flavour.
It cuts the other way for delicate tea
For green, yellow and white tea the lower ceiling is a small convenience rather than a problem. These teas want water well off the boil anyway, and starting from a lower maximum means less waiting and less risk of scorching the leaf through inattention. If you have moved somewhere high and found your green tea suddenly more forgiving and your black tea flatter, this is why, and it is one of the few brewing variables that is genuinely out of your control.
The kettle problem
There is a practical wrinkle worth knowing. Ordinary kettles cut out when they detect boiling rather than at a set temperature, so they will happily stop at whatever the local boiling point is — that part works correctly. Temperature-controlled kettles set to a target near boiling can behave oddly at altitude, because the target may sit above what the water can actually reach, in which case the kettle keeps heating and the water simply boils. That is not a fault, and it is a good reason to trust the boil rather than the display when you are high up.