Mark It Down WordPress!
by Wasseem Khayrattee on WordPress.org
Helping you write posts/pages in plain-text Markdown syntax, while switching-off Visual Editor. It sits on the shoulders of a giant, namely Jetpack.

This screen shot description corresponds to installation step 3, which instructs: Enable Markdown for posts & pages, by manually checking the checkbox at: Settings > Writing > 'Use Markdown for posts and pages'
This is a simple plugin to help you write posts or pages in plain-text Markdown syntax. And even allowing plaint-text Markdown for comments as well. It will also switch-off the Visual Editor, as you don’t need distractions. This plugin sits on top of a giant’s shoulder, namely Jetpack.
This plugin is also installable via composer, see: https://github.com/7php/MarkItDownWordpress
Roadmap
- By default, toggle the checkbox to true for Settings > Writing > ‘Use Markdown for posts and pages’
 - By default, toggle the checkbox to true for Settings > Discussion > ‘Use Markdown for comments’
 - In plain-tex editor mode, remove the short-formatting buttons, e.g [b, i, link, b-quote, del].. etc. Make the editor, distraction free.
 - Make an effort to provide Markdown buttons for the equivalent of [b, i, link, b-quote, del], just like bbcodes.