[belter]

S3 buckets were never public by default. From the link you posted:

"...Amazon S3 buckets are and always have been private by default. Only the bucket owner can access the bucket or choose to grant access to other users..."

The feature and announcement you linked was about making active an additional safety feature that would block them becoming public. Even if you intentionally ( or accidentally ) configured them with public access.

The well known accidents in the past, of Facebook or the Pentagon having private data in public S3 buckets, I can only attribute to the modern practices of self-paced learning, skipping videos on Udemy courses or deciding formal training is no longer necessary because I can Google it...