"1: text_note: the content is set to the text content of a note (anything the user wants to say). Non-plaintext notes should instead use kind 1000-10000 as described in NIP-16."
https://github.com/nostr-protocol/nips/blob/master/01.md
markdown is plaintext so it is technically allowed
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I've created an issue to get clarification on this point
https://github.com/nostr-protocol/nips/issues/297
