The singular indefinite they was used for a person of unknown gender for a very long time. Consider:
"Someone left their wallet."
If you knew that it was Whiteshark's wallet, you'd say: "WhiteShark left his wallet."
The modern nu-singular they is a definite they: "WhiteShark left their wallet."
The "akshually, singular they has always been used!" crowd is purposely conflating these two.
If you're having trouble understanding the difference, know that this is something that tripped up a bunch of style guides, and not even for pozzed reasons β chicago styleguide went back and suggested
against using it in the 'modern' way after first recommending to do so.
https://www.chicagomanualofstyle.org/qa ... q0018.html
The Builder, 1927, Volume 4
The person who lost their keys is specific, but not definite.
The reason you'd see "he/him" used in, for example, the AD&D PHB was because the reader is specific and definite. This is despite the author not knowing who the reader is, exactly. It is specific to the reader's context.