Originally called Twitter Notes, and now called Articles, it's currently being tested among a small group of users, according to Twitter's website.
Musk may not remember this (I don't think that many people do) but Facebook also had a feature called Notes.
Facebook Notes was introduced in 2006.
Basically, it was the exact same thing as Twitter Notes, but 17 years earlier.
All in all, it doesn't seem like the opportune time to roll out a long-form writing feature.
Persons:
Elon Musk, it's, Musk, I'd, tweeting, Mark Zuckerberg
Organizations:
Facebook, Twitter, YouTube