So you have to pick your lock-in for a Markdown processor? Once you step outside of CommonMark, nothing is compatible. Compare that to AsciiDoc, reStructuredText, Org mode… these support metadata as a first-class feature (as you would expect from almost every other creative format: *.ogg, *.webm, *.odf, *.png, *.svg, *.html, etc.). Choosing such tools makes it a) harder to migrate to another tool & b) difficult/impossible for other tools to render it properly. You can skip that nonsense by just choosing a better format.