When watching the Vintage Space video Apollo’s Flammable Oxygen Atmosphere Explained, I started to think: was there ever any spacecraft proposed with nitrogen-oxygen atmosphere, but without a system for nitrogen replenishment?
Nitrogen is not used by humans for breathing, so, theoretically, the content of nitrogen in the cabin should remain constant, unless there is a leak or you need to depressurize the spacecraft (i.e. for EVA).
Thus, it seems that such atmosphere would combine reduced flammability with the engineering simplicity of oxygen-only life-support system, at cost of having a (big?) problem in case of depressurization event.
Has anyone ever seriously proposed such solution for any manned spacecraft?