This isn't _entirely_ accurate. Gender binary may be an oversimplification or diminutive concept, but the idea that it "never existed" is a little extreme. It would almost be like saying religion never really existed because it's a figment of human imagination. In that sense, sure, binary genders are a bandaid over a complex subject, but it does exist. In fact, it is quite literally birthed from the genesis of more than a few religious ideologies. It exists just as much as the religions that buy into them exist.

Anyway back on topic:

Open field text boxes come with a practical cost of being much more difficult to aggregate, and let's be honest, the gender data is probably not important enough for GitHub to expend too much energy on. The more costly you make the data to mine, the less likely that the data will be used effectively.