> The thing about singular they is that it only really became "perfectly good usage for longer than people think" within the last few years, well after the heyday of Usenet.

The singular "they" has been prescribed in manuals of style since the 1700s[0], continuously through the 20th century. That certainly predates the heyday of Usenet.

[0] https://en.wikipedia.org/wiki/He_(pronoun)#Gender