Half Life 2: Episode One 1024x768 with 8xSSAA
This was rendered at 4096x3072 (That's ~1.5x more than a native 4k resolution) then downscaled to 1024x768 and losslessly captured.
It was then losslessly upscaled through FFmpeg to 2880x2160 (non-integer) with a nearest neighbor filter to be YouTube friendly.
Achieved with ValveSoftware's GameScope for Linux. Special thanks to @RuanEvangelista