Fred G. Black wrote:

This isn't a concern for
en-route operations since everybody's altimeter has the same "error". It
can be a concern for terrain clearance in the mountains or in some cases
instrument approaches in cold weather since the altimeter reads higher
than the plane actually is.

No concern, either. When IMC, there is a safety margin built into the
approaches, and when VMC, there are windows in the copckpit.

Pneumatic altimeters are reliable, independant of any infrastructure and
work without electricity.

Stefan