I have been made aware of a dynamic subsetting font service in Japan: https://fontplus.jp/service
which notes
使う文字だけを読み込む効率的なフォント表示。ページが重くならないので、高品質なページをすばやくユーザーに届けます。
(Efficient font display that reads only the characters to be used. It doesn't make the page heavy, so it delivers high quality pages to users quickly.)
I have no other details about this.