For an issue on v3 API, use ?expand=renderedFields. For comments, use ?expand=renderedBody. This will give you HTML.
You can also try using the Confluence converter to convert between various formats.
Unfortunate there is no way to get plain text that doesn’t include markup (e.g. [~accountid:1234abcd]). Very annoying when integrating with plain-text systems. Why can’t we easily get it as @Display Name?
“Rendered plain-text” is what we want for integrating with external systems that do not support HTML - essentially, identical to the HTML rendered view, but with the HTML tags stripped. Hell, give us a .stripHtml in Jira Expressions, Smart Values, and any other expression systems you decide to create instead of re-using existing systems (WTH are SVs different than JEs?!?!) and we can do it ourselves.