There is no way to make breathable oxygen on Mars
The old claim turned out to be wrong and was rejected by evidence.
TaughtMars's atmosphere is 95% carbon dioxide and almost unbreathable. Any human mission would have to carry every molecule of oxygen, for breathing and for rocket fuel, all the way from Earth, because there is no way to produce it there.
NowOn 20 April 2021, a toaster-sized instrument called MOXIE, riding on NASA's Perseverance rover, pulled oxygen out of the Martian air, splitting carbon dioxide to produce about 5 grams of breathable oxygen. It was the first time a natural resource had ever been harvested on another planet.
What actually happened
MOXIE works by solid-oxide electrolysis: it takes in the CO2 that dominates Mars's atmosphere, heats it, and strips off oxygen atoms, venting carbon monoxide as waste. Over the following two years it ran successfully more than a dozen times, in day and night and across seasons.
The honest scale: MOXIE was a proof of concept producing roughly what a small tree makes, nowhere near enough to breathe or to fuel a return rocket, which would need a machine hundreds of times larger. It was designed to prove the principle, not to supply a base. But the flat claim it tested, that oxygen simply cannot be made on Mars, is no longer true.
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Who was taught this
Still standard through 2021, so anyone who finished school between 1950 and 2021 learned the earlier version.