Martin from GitHub here. Sorry about that message - team are rolling out an update to change the text and should be fixed soon. In the meantime if you ignore that message and downgrade from a legacy plan to Free then you will retain access to your private repositories.
>>martin+(OP)
thanks for the fast and reassuring answer, I appreciate it. I'll wait until that message goes away, I can't risk losing my private repos.
>>martin+(OP)
Is the system supposed to be charging for outside collaborators on the Team plan still? The language makes it sound like those should be free now.