Pledge:

A solemn binding promise to do, give, or refrain from doing something

It sounds a bit dramatic and self-sacrificing. That has its good points - a 'pledge' might get more attention than a 'statement'. But I think it will also be harder to get people to sign if it sounds like they're promising to do something risky.

I favour publicly calling this a 'statement', with the implication that we're just highlighting what we were already going to do. The common base level should be "we won't support Python 2 beyond 2020", which doesn't feel controversial or dramatic. Then individual projects can add details to go further, e.g. bugfixes only for Python 2, or ending support before 2020.