PagePlus X9 already has the "import text" feature that works for a variety of document types (including .odt and .docx), Markdown files are comparatively simple, so an importer of this nature shouldn't be an overwhelming request.
As for InDesign, I don't know if it supports markdown, but it also has test import capabilities. In fact, in InDesign, the intended workflow seems to be for the designer to link external files, and have authors edit the external file directly, rather than messing with the design. I assume that this kind of functionality is a small part of why PagePlus is being deprecated in favor of Affinity Publisher.