Hi! Birdwatch ML Engineer here-- these context notes are from Birdwatch. They are written and rated by users, and the notes are only added as context to Tweets if they are rated highly enough by multiple raters who have tended to disagree in the past. The core algorithm is open source as well as all of the data, and there is lots of public documentation about it too:
https://twitter.github.io/birdwatch/
https://twitter.com/birdwatch/status/1585794012052611076?s=2...
https://arxiv.org/pdf/2210.15723.pdf