Sure, clients can do anything, that includes things that are much worse than Markdown. No protocol will survive if everybody starts to do awful things -- or if a single dominant client does "embrace and extend" techniques. Nostr is very fragile.
However the Markdown issue often happens because client makers want a way to automatically turn URLs into links, so they add Markdown libraries, not realizing the evil that would bring, so what I am doing is telling them.
