NIP-01 states:
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.
Markdown is plaintext so should be allowed. Paging #[6] for clarification.
#[7] has stated several times against using markdown in kind-1 notes.
Markdown syntax for links, footnotes and images is definitely not plaintext. The rest is tolerable. Please don't break it for all other clients or force everybody to support a cumbersome and heavy syntax with all the garbage that comes with parsers, plus the indeterminism as there are multiple flavors or Markdown, it's an endless shithole.
Did you know Twitter doesn't have Markdown support?
