Markdown Quote with Onebox Header

Continuing the discussion from,

I wanted to suggest that quotes could be handled without an extension to markdown syntax: a new route could be created for quote links, with a special onebox engine that renders that link identical to existing quote box headers with no padding below the onebox, such that the following:
>The number 1 issue with using CommonMark here is lack Discourse specific extensions.

would render exactly the same as a quote using the current bbcode syntax does. If the quote were expanded by clicking the chevron however, the full post would show, as it currently does, except it wouldn’t clobber the markdown quote.

I understand that currently, links to posts with restricted visibility do not onebox. Those links, in quote format, could onebox to a quote header with no chevron. By the way, I have found a related problem with quoting restricted visibility posts:

Keep in mind that you still have to keep compatibility with existing written text.

I would expect that, if I am understanding what you mean by compatibility correctly, that the change to standard markdown would be a breaking change, and that a solution could be to add a ‘parser version’ column to posts to identify the posts that should never be rebaked – problems with those posts can be solved by editing, to trigger a fresh bake with the new parser.