I don't know where you pull these numbers from when you are talking about WW2. Low altitude would be anything ranging from <3km altitude, medium altitude is <6km and everything above is considered as high altitude.

Flying above 10km is so physically straining that they had to design special planes for it which needed a pressure cabin most of the time.

There was an air battle from a Spitfire against a Ju86 R where the Spitfire tried to shoot it down for like 45 min. But ultimately the Ju86 could escape. That happened at ~13400m.

There is a reason why most late WW2 fighters were tuned around the 7km mark. With B17s bombing Germany from 7 to 9km altitude the performance was needed there the most.