https://techcrunch.com/2023/04/13/twitter-introduces-10000-character-long-tweets-for-blue-subscribers/

Twitter has introduced a new feature that will let Blue subscribers post 10,000-character-long posts — as if the social network is trying to compete with a rival newsletter platform. Along with this, this Twitter has also added support for bold and italic text formatting.

In February, the social network introduced 4,000-character-long tweets for Blue subscribers to encourage people to publish longer posts instead of threads.