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Re:Slack’s new WYSIWYG input box is terrible by ansgri on Thursday November 21, 2019 @04:32AM (#21592604)
There's at least Redmine. Uses Textile by default, but there were plugins for Markdown.
Re:Slack’s new WYSIWYG input box is terrible by wila on Thursday November 21, 2019 @06:34AM (#21593136)
Redmine nowadays also comes by default with Markdown.

Menu Administration -> Settings -> General tab

There's a drop down for Text formatting which you can set to Markdown.