English frequently uses they to refer to a singular person.

It's been common place all my life and that's going on six decades now.

'Merkin English, however, likely has prescriptive rails and a lingering shrinking horror stemming from the inappropriate touch of Webster.

> those old paper books are more accurate guides to how people use it than their ideologically-driven on-line counterparts.

Exactly - see the full OED (Oxford English Dictionary) entry on the long historic use of 'they' over seven centuries and the many ways in which it has been (and continuous to be) used contemporaneously both as singular and plural.

Using archaic latin terms doesn't really strengthen your argument for linguistic evolution.

However, for certain using plurals for individuals of unknown gender is hardly rare, there are hundreds of examples in literature a quick Google away.

It has become (somewhat) popular in the context particularly of discussions on trans rights and 'misgendering' to take the position that the singular they is somehow ungrammatical because people don't want to use it to refer to people whose gender they believe is not unknown.

But it seems to me as though that argument is actually not related to the current discussion.

Here we are simply discussing whether 'they' can be used to refer to a person whose gender is unknown to the speaker at the time, for which there is ample historical, literary evidence that this is commonplace in English, as well as the anecdotal evidence from sibling comments.

I'm a native English speaker who grew up in the US speaking English through the 80s and 90s. Use of 'they' as a gender-neutral third-person singular was common long before the ideology you're mounting this pathetic attack against.