Even though we aren't responsible for any third-party support or lack thereof, MyOpenID affects Stack Exchange users, so yes, it is a good idea for us to warn people that they might want to move away from it.
I'm exploring what the best way to reach users who only access Stack Exchange through MyOpenID. We will prompt them in some manner to add an alternative login method to their accounts and let them know that MyOpenID could go down at any time, for an unknown period of time.
UPDATE: I've posted an announcementan announcement here on MSO about the fact that MyOpenID is essentially going away. That will draw some attention to the matter and provide detailed instructions on how to add a new login method to your account. We'll be sending notifications soon to users who only have MyOpenID logins on their SE accounts.