In the description box of a Flow, new lines and other formatting are not respected. This is basic functionality in 2019, and is misleading, because newlines, at least, are supported in the edit box that pops up on the right-hand side of the screen when editing the description. To be specific, I'm talking about when you click on a Flow from the My Flows or Team Flows pages and can view the title, description, 10 most previous runs, owners, and a few other pieces of meta data.
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We do not plan to allow the customization of the formatting of these emails. However, you can use rich text in the Approvals actions themselves (see here), and we plan on adding custom options to these actions (see here).