Altimeters in aircraft work on pressure only. They have no electronic components. They are manually set to the altitude of the airport on departure as their readings are affected by local air pressure (weather) and that needs to be compensated for. They cannot be affected by 5G. Airplanes are still safe, modulo the vaxx status of the pilots.

I suppose on the big jets they might have radar altimeters in addition to pressure altimeters that if poorly designed could be interfered with by 5G but most folks designing radars are not stupid and the radios using digital signal processors are pretty darn flexible these days. New radio/radar is a software change.