(Actually, to be more specific, a YAML file with no directives, explicitly-signalled start-of-document-content, and followed by a second null document. I will note that frontmatter syntax is not specified; the non-normative Appendix B is the only place that suggests it means prefix and suffix --- lines. And no, frontmatter is not part of Markdown, or CommonMark, and is in fact incompatible with both. And it’s invalid YAML too, the end-of-frontmatter line should be ... to indicate end of document without starting a new document.)
Looks like unless something better comes up, we'll be stuck with it for a while.
I find markdown useful for repo-specific conventions, especially skills.