When I exclusively used WordPress’s classic editor and Jetpack’s Markdown module (or Easy Markdown), syndicating Markdown to, e.g., Mastodon, was easy. (Both plugins store unparsed Markdown in post_content_filtered, so I’d cross-post that.)
Now that I spend more and more time in the block editor, I have this issue where blockquotes, once syndicated, are no longer recognizable as (block)quotes—I strip away HTML before POSSEing, and there’s no > or nothing in the original post.
Might eventually solve this by converting (all) HTML to Markdown first.