Well, there's this: https://commonmark.org
But everyone seems to have their own slightly different flavor, either pre- or post- that "standardization".
Notably, Github's superset:
Given Github's owner, it's right out of their Embrace, Extend ... playbook.
GFM predates microsoft’s acquisition of github by one year as a formal spec and six years as an informal one - but ms hardly has a monopoly on embrace-extend-extinguish.