It believe was removed from NIP-01 quite awhile ago. It's in NIP-23 though.
Discussion
Removed ?? Why ? .. I mean the whole point of posting on nostr is it takes markdown files ..
I respectfully disagree that’s the whole point of nostr 😂
Hyperbole :-) .. disagreement accepted .. but it is a big deal .. isn't it ?
Markdown is html .. html is Browser .. nostr clients should parse html ( and thus markdown ) .. its like a no brainer ..
For some, definitely. I’m in the #tardstr sector though. Idk about markdown really, aside from it meaning that punctuation/symbols can turn text into different sizes and formats
When we talk about what nostr is “for” beyond freedom tech/censorship resistance and the like, i think we have one of these situations 
#tardstr
I'm not actually sure if it was without checking file history but I thought I remembered seeing it being in NIP-01 at one time. I guess it's because nip-01 is just supposed to be plain ole notes/posts
After reading through NIP 23 ..it kinda makes sense ..
Format
The content of these events should be a string text in Markdown syntax. To maximize compatibility and readability between different clients and devices, any client that is creating long form notes:
MUST NOT hard line-break paragraphs of text, such as arbitrary line breaks at 80 column boundaries.
MUST NOT support adding HTML to Markdown.
But why remove this from NIP 01 ? Is it too much performance tax ?