Learning Prismic right now and it really lacks markdown support, even Contentfiul has it. It even lacks just a plain text field with newlines. Which I need to use to create markdown.
Welcome to the Prismic, and thanks for reaching out to us.
Indeed, Prismic doesn't have a Markdown feature. Our editor team is already tracking it as a feature request but I don't have an ETA on when this will be implemented.
There is a workaround to create Markdown. Please follow the article: How to add markdown in a Prismic field.