I keep seeing people discourage others from producing game material for sale.

Often, the word used is “commodifying.”

A commodity is a good that is interchangeable with a good of the same type. If you grow the conventional form of field corn and I grow the conventional form of field corn; there isn’t any significant difference between a bushel of my corn or your bushel of corn.

If I grow a landrace variety of sweet corn using organic production techniques, pick it early in the morning for sale that day, and only when it is in season, that is not commodification. That is an entirely different thing. I hesitate to use the word, but you might call it, artisanal.

I suppose if a self-published game designer produces something that looks, feels and plays just like what a major game company is producing, you might call that a commodity.

I don’t see much of that going on in the OSR. In fact, quite the opposite.