Several people have reported to @Airbnbhelp that they have received an email regarding new Terms and Conditions and asking users to login in and agree to them. THIS IS A PHISHING EMAIL AND NO ONE SHOULD RESPOND TO IT. The scammers are getting very clever. If you have received and responded to this email, contact Airbnb right away!!
I too received the pop up and agreed to it. Since the pop up, unlike the example described in this conversation thread, did not ask for my log in information, I feel that it is safe. Nonetheless, you are smart to be wary.