If atmospheric nitrogen dropped by half, I'd imagine the effects would be pretty significant for aircraft, birds & internal combustion engines. Rocket launches would become more efficient (less drag), but reentry would be more challenging, w/ less overall drag & proportionally more oxidizing effect when aero-braking

Most engines would probably run hotter & weaker w/out as much nitrogen to use as a working fluid (maybe we'd compensate for this w/ water injection & get even better performance though?)

The speed of sound would decease slightly, while the speed needed to generate lift at a given altitude would increase. This would definitely affect airplanes, e.g. a plane that can cruise at 15k feet but not 30k feet might not be able to reach 15k feet anymore