Comment by trealira

The word "they" being used in a singular manner is not something invented by transgender people. It has existed for a long time in natural conversation when talking about someone indeterminate. You can see it in examples like "Has anyone forgotten their coat?" or "Someone who's bought their own house would know." Activists couldn't hope to make people talk that way if they didn't already.

You can see it randomly used in old writing, too. For example, I just found this example of the word they being used in a singular fashion in Jane Eyre.

I observed when any one entered or left the apartment: I could even tell who they were; I could understand what was said when the speaker stood near to me; but I could not answer; to open my lips or move my limbs was equally impossible.

It was not uncontested, however; 19th century grammarians prescribed that people use "he" for indeterminate persons, and you will see this a lot in old writing, too.

Finally, I will say that transgender people have existed for longer than the 1970s. For trivial evidence, you can see how Christine Jorgensen got a sex change in 1954, making headlines as "Former G.I. Becomes Blonde Beauty" in a newspaper.