POST /gist/{gist_id}/forks has the following note in its description:
Note: This was previously /gists/:gist_id/fork.
The endpoint in question is not part of the official documentation, and while the internet does have traces they seem to be rather old. I can only guess that the endpoint was /fork and eventually got renamed /forks years ago, keeping the note around seems at best useless and at worst confusing. It doesn't even use the openapi description style.