Actually, you might be surprised. There was kind of a drop off in the trend of increasing combat altitude. Notice most fighters are optimized for performance between 23-30k, instead of 30k+, even fighters from late 1944 to 1945, like the K4, D9, Spit 14 and 16, the P-51D (though this was late 43 to early 44), the Tempest, F4Us (all of them), and the Russian and Japanese fighters are optimized for even lower!