After merging the CommonMark patch #32424, Redmine will contain an @HTML::Pipeline@ implementation limited to this new format. It makes then perfect sense to extend the new concept to text formatting generally.

The current implementation looks like:
!Text_Formatting-Redmine-4.2.png!

While the new Markdown (CommonMark) implementation is designed like:
!CommonMark-HTML_Pipeline.png!

Migrating text rendering to @HTML::Pipeline@ should provide several benefits:

• Better separation of concerns.
• Cleaner, safer and DRY code.
• Individual formatters can be freed from excessive restrictions that were introduced as XSS prevention. Especially the absolute prohibition of HTML tags can be pain in specific cases.
• More consistent behavior among formatters (e.g. HTML is currently escaped in Textile, but dropped in Markdown),
• Controlled level of security without the need to rely on individual formatter implementations that often use poorly maintained libraries.
• More flexibility for Redmine developers and Redmine plugin developers. Adding some hooks would be nice and would allow e.g. implementing @mermaid@ charts as a custom syntax highlighter - in the same way as GitLab.

Some inspiration "can be found in GitLab sources":https://gitlab.com/gitlab-org/gitlab/-/tree/master/lib/banzai/pipeline, but our pipelines would probably won't need to go that complex.

If there were no other timing considerations, it would make sense to implement this together with Redcarpet removal.