DEVONthink uses MultiMarkdown 6.x which doesn’t support this (yet). As far as I known this is a Markdown extension introduced by github and one of Markdown’s shortcomings is indeed that there’s no official specification.
I get the sense that in the long-term CommonMark will emerge as the standard - any chance of supporting that?
Among other reasons, you have noted the popularity of Discourse as evidenced by this Forum. Discourse has been a strong force in advancing the use of Markdown - and they clearly support the CommonMark standard.
Slack style emoji isn’t a Markdown convention. DEVONthink on the Mac has support specifically for it. However, Markdown documents support unicode characters so you can add emoji with the emoji keyboard or Character palette.
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