Twitter added language to their Terms of Service that allows them to “limit distribution or visibility of any Content…” (in other words: shadow ban). This was noticed about a month prior when Twitter released the new ToS that would come into effect on January 1, 2020. The full sentence reads:
We may also remove or refuse to distribute any Content on the Services, limit distribution or visibility of any Content on the service, suspend or terminate users, and reclaim usernames without liability to you.
It was suspected that Twitter was doing exactly this as early as February of 2016 when Breitbart receieved reports from two different sources. Two years after that in January of 2018, Project Veritas released an undercover report on Twitter which revealed even more about the shadow banning practices.