https://soupault.net/ is about using plain HTML, but doing index pages, RSS feeds and so on from that. You even get away with not having frontmatter, because CSS like selectors allow those meta pages to retrieve title, date etc. from the HTML pages.
As a learning exercise, I wrote my own little SSG in ocaml, and man I forgot how nice a language it is. Tooling is still a bit rough but lots better than it used to be
