I think the docs at

• https://github.com/jekyll/jekyll/blob/master/docs/_docs/configuration/markdown.md
• https://jekyllrb.com/docs/configuration/markdown/

are outdated.

At least the option gfm_quirks:true does not work. It says:

       Jekyll Feed: Generating feed for posts
  Conversion error: Jekyll::Converters::Markdown encountered an error while converting '404.md':
                    Invalid type TrueClass for option gfm_quirks
bundler: failed to load command: jekyll (/Users/tobiasjordans/.asdf/installs/ruby/3.0.0/bin/jekyll)

This is in line with https://jekyllrb.com/news/2020/08/05/jekyll-3-9-0-released/ which I read as: GFM will not work out of the box.

However, bundle add kramdown-parser-gfm and using

kramdown:
  input: GFM

from https://www.markdownguide.org/tools/jekyll/ does work.

It might be, that the page still references kramdown 1.x, not the new 2.x?

Suggestion

Update the docs.