How is paint embrittlement countered in high altitude aircraft? There are cases of aircraft that are not designed for high altitudes that can still fly there and as a result the paint on these aircraft "falls off".
Although most aircraft have paint that is chosen to work up to their maximum flight altitude (that is mainly limited by the engine and by aerodynamics), sailplanes are an example of aircraft that do not have a fixed maximum flight altitude. As long as thermals push these aircraft higher, there are few limiting factors for their maximum altitude, as the necessary lift and thus velocity at high altitudes is not as significant as other aircraft since they have a large wing area and low wing loading.

My question thus is not centered around the aerodynamic limits and answers such as "it is selected to work up to its maximum flight altitude" but rather around the process of the selection of compatible paints. It may include design and selection considerations for civil aircraft at 10.000m AGL and for high altitude military fighters. Does the paint over the aircraft surface change due to additional influences e.g. front edge heating?

If necessary, an overview of the different paint types used (at certain altidudes) would be appreciated.
There are 3 critical areas one may wish to address with paint:  the formulation of the paint, how it is attached to the substrate, and the properties of the substrate.

The substrate may expand, contract, bend or twist with heating, cooling, or stress.

The attachment of the paint to the substrate is crucial to durability.  Surface preparation includes cleaning, deposition of a chemical coating such as chromate, or primer.  This is a make or break step that should not be ignored.

Finally the paint itself.  Bare bones paint formula is solvent, pigment, and resin. The solvent evaporates, leaving the pigment (color or shade) and the resin, which is essentially the coating.

Factors such as temperature, flexability, hardness, weather and UV resistance must be considered.  The closer the properties of the paint are to the substrate the better.

I would find out what the airliners use.