Upon logging in, I noticed it suddenly downgraded me to free although billing is current. People are reporting they also lost Plus subscriptions and all chat history.
You're probably not downgraded, but rather the UI defaults to free plan, and once the API confirmed you're subscribed, the state switches to the "Plus" one. Same for chat history and more. Just give them moments to restore the backend and it'll come back surely.
Personally I saw "You've been flagged for suspicious traffic" for a few moments before the whole thing went down. I'm not using any extensions or anything extra, just raw vanilla ChatGPT Plus and been using it almost daily since ~1 week after GPT4 launch, with no malicious usage at all. So guessing that was also a temporary message because of some state shenanigans.
Personally I saw "You've been flagged for suspicious traffic" for a few moments before the whole thing went down. I'm not using any extensions or anything extra, just raw vanilla ChatGPT Plus and been using it almost daily since ~1 week after GPT4 launch, with no malicious usage at all. So guessing that was also a temporary message because of some state shenanigans.