VxWorks has been around forever, and while Linux likely leads in raw unit count now due to massive constellations like Starlink, an incredible number of legacy deep-space missions are still chugging along!

I heard Curiosity only has 1% of the availble memory on-board and it's still doing science. Memory mapping and not requiring an MMU probably helps with that.

https://spectrum.ieee.org/curiosity-rover-jpl-mars-science