Mind you, what would there be to negotiate? I find it extremely unlikely Whittaker would stay after her contract is done, that doesn't seem to be how things go with modern Doctors. Even Tennant and Capaldi, life long fans who considered becoming the Doctor their ultimate goal as actors were out the door the minute their contracts were up.
I guess it's possible her finale might be in the holiday special which is not considered part of the three season contract, which would require negotiations to get her in it, as was the case with Smith and Capaldi.
Precedent. Every other time tabloids reported rumours of Whittaker's departure, there was an official denial within days, a week at the longest. The lack of a denial this time is notable.