well you forget that racists are proud of the term white while transphobes hate the idea that they aren't the only type of normal.
As I told a friend however, this is secretly hilarious for a couple reasons.
Firstly becuase Musk isn't the CEO anymore, doesn't get to make these decisions anymore, and people are already reporting that twitter has been rejecting people reporting users who use the term cis.
Secondly because if cis and cisgender are forbidden on the platform then it means, logically, nobody can identify as cis or cisgender on the platform. If cis isn't an allowed identity on the platform then it means that twitter's owner has in a way declared that twitter is only for trans people as its still an officially supported identity.