I have a slightly older 2016 rulebook. Looks like they changed displacement, allowing up to 800cc twins w/o Desmodromic valves and 750 with minor limitations.
EDIT: looking at them now. F2 allows up to 800cc liquid cooled twin, but it did not before. That seems to be the main thing. Adds some detail about Desmodromic valves that wasn't there before.