https://fossil-scm.org/home/vdiff?from=trunk&to=markdown-extra-emph-chars

Text styling:

subscript: ~text~, e.g. H~2~O
supscript: ^text^, e.g. E=mc^2^
++inserted text++
--deleted text--
==mark==
~~strikethrough~~

Questions:

1. Are the proposed enhancements in the CommonMark Spec?

2. Are the proposed enhancements supported by other popular Markdown rendering engines, such as on GitHub?

All of of them I think not present in CommonMark.

1. strikethrough, via double ~: github, gitlab, forgejo (which uses goldmark), discount, pandoc, VS Code (which uses markdown-it library), marktext
2. sub-, supscripts, via single ~ and ^: github, gitlab, forgejo - no; lowdown, discount, pandoc, VS Code; asciidoc
3. highlighted text, via ==: again github, gitlab, forgejo - no; VS Code; in asciidoc via single #, in pandoc via [Mark]{.mark}
4. inserted, deleted text via -- and ++: libsoldout, VS Code (only ++); gitlab uses slightly different syntax {+ additions +} or [- deletions -]

On a sightly related topic (the fact that the common spec in insufficient for more elaborated, but common enough documentation tasks), Jhon McFarlane is proposing djot, that tries to be familiar enough to Markdown users while learning from Markdown's limitations of the format and the creation of parsers for it.

Things like admonitions, subscripts, superscripts, additions and deletions are part of that format. I think is evolving in a good direction and is a format to have in the radar, regarding conversations like the one on this thread.