Markdown for large text fields (private preview) - Azure DevOps Blog
Briefly

The long-requested feature of Markdown support for large text fields in work items has entered a private preview phase. Users can opt to switch from the default HTML editor to Markdown for existing and new work items. However, once converted to Markdown, reverting to HTML is not possible, and complex HTML may not convert perfectly. Existing users can convert their field content, while new items will default to the last chosen editor preference, remembering whether Markdown or HTML was selected.
After the conversion takes place, the work item will display the text in Markdown format whenever you edit it.
Once you convert a field to Markdown, there's no way to revert it back to HTML.
For new work items, the experience is similar, except there is no existing data to convert.
The Markdown editor is only available in New Boards. Your organization must be fully switched over.
Read at Azure DevOps Blog
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