What is the typical cruising altitude for a civil airplane?
I think there are already plenty of answers here to give you a clue:


How do pilots decide what their cruising altitude will be?
why are regional cruising altitudes higher than those of international much longer flights?
How can you find the best altitude for range or speed?
Why are many jet aircraft designed to cruise around FL350-370?


Normally, atitude restrictions give a top limit. A minimum altitude is hard to enforce when one engine fails - then the aircraft can only continue at a lower altitude (typically around 15,000 ft for a two-engined jet) or the cabin pressurization fails (now the aircraft needs to continue below 10,000 ft).

This is speculation, but maybe the 26,000 ft limit is caused by information that rebels in the West Sinai have gotten hold of shoulder-launched anti-aircraft missiles. Those have a fairly low upper limit to their effectiveness.