Reputable S3 game buckets usually host only open-source or publicly available games. They lack pop-up ads, which are the primary vector for malware on shady "unblocked" sites. Because it is static HTML, there is no server-side script trying to inject a virus into your computer.
Sites like CrazyGames or Poki provide large libraries, though they are more likely to be flagged by filters than raw S3 buckets. Additional information includes: Unblocked-games.s3