While it is within Twitter’s policies to advertise crypto and NFTs (Non-fungible tokens) on the platform, these sorts of advertisements are restricted. In all cases, the advertising on these types of crypto-assets requires prior authorisation from Twitter.
The advertisement of many blockchain products is, indeed, banned by Twitter, however, NFTs aren’t listed specifically as banned. Additionally, rules regarding the advertising of NFTs on Twitter vary from country to country. The specific regulations for different countries can be found here on Twitter