Extensions broken by #1260:

• mdx_gh_links
  Latest release fails with Markdown 3.4 github-links#10
• mdx_truly_sane_lists
  markdown no longer exports version radude/mdx_truly_sane_lists#11
• markdown_include
  Breaking my site with latest updates  cmacmackin/markdown-include#33
• plantuml-markdown
  Config value: 'markdown_extensions'. Error: extendMarkdown() missing 1 required positional argument: 'md_globals' mikitex70/plantuml-markdown#67
• mdx_include
  python-markdown's version_info is deprecated neurobin/mdx_include#7
• lightgallery
  Failed loading extension "lightgallery" with mkdocs material g-provost/lightgallery-markdown#12
• featherlight
  https://github.com/it-novum/mkdocs-featherlight
• fontawesome_markdown
  Plugin causing issues with mkdocs bmcorser/fontawesome-markdown#17
• mkdcomments
  Extension broken by markdown >=3.4.0 ryneeverett/python-markdown-comments#7
• mkautodoc
  Address deprecation of markdown.util.etree lovelydinosaur/mkautodoc#33

Other projects that aren't purely extensions:

• pweave
  Adjust for API changes in Python-Markdown 3.0 mpastell/Pweave#167
• cmdtest
  https://salsa.debian.org/python-team/packages/cmdtest/-/blob/master/debian/patches/0001-Fix-for-new-version-of-markdown.patch

(Checkmark means that a fix is released)
I'll keep this list updated as I become aware of more.

See also: instructions for MkDocs - feel free to post there.