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
Discussion
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 ?