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 -]