(for your own aircraft, not other players, although maybe that too, (it should be optional) At present, it’s set to maybe 2km, for small planes, and 10km for bigger planes.
It wouldn’t make much difference CPU wise if your plane could stay visible for a few more miles, it really breaks immersion for me to see my plane disappear so quickly when watching it through the mountains or go up into the clouds.
In my opinion your aircraft shouldn’t ever disappear in a ‘render distance’ way.
It should only disappear when further away than the meteorologic visabilty distance or the aircraft would be smaller than half a screen pixel when rendered. (unless obscured by something of course)