That's true now. Used to be the case that they'd recommend random or high-entropy parts of the keys go at the beginning to avoid overloading a shard as you described above.
From [0]:
> This S3 request rate performance increase removes any previous guidance to randomize object prefixes to achieve faster performance. That means you can now use logical or sequential naming patterns in S3 object naming without any performance implications. This improvement is now available in all AWS Regions. For more information, visit the Amazon S3 Developer Guide.
From [0]:
> This S3 request rate performance increase removes any previous guidance to randomize object prefixes to achieve faster performance. That means you can now use logical or sequential naming patterns in S3 object naming without any performance implications. This improvement is now available in all AWS Regions. For more information, visit the Amazon S3 Developer Guide.
[0]: https://aws.amazon.com/about-aws/whats-new/2018/07/amazon-s3...