Right, but it's worth noting that those supersets are very common. I'd say it's actually less common to run into a "barebones" Markdown implementation these days than to run into a GFM or close-to-GFM one.

Yes, for example in Hugo [0] I can do this:

This is some text with a footnote.[^1]

[^1]: And that's the footnote.

[0] https://gohugo.io/