> First, R2 is not 100% compatible with the S3 API. One notable missing feature are data-integrity checks with SHA256 checksums.

Maybe it was an old thing? The changelog [0] for 2023-06-16 says:

"S3 putObject now supports sha256 and sha1 checksums."

  [0]: https://developers.cloudflare.com/r2/platform/changelog/#2023-06-16