GNU Boot seems to have sent it not for anything in Libreboot itself, but to adress a web page with self-proclaimed “unofficial” GNU Boot releases. They wanted it to stop proclaiming to be “unofficial GNU Boot” releases. Understandable, if a bit antagonistic.
EDIT: Courtesy of user jbit¹, this is the aforementioned web page:
<https://web.archive.org/web/20230719185342/https://libreboot...>