S3 _does_ support CAS on copyObject [0].

Cloudflare supports CAS on copyObject and on putObject [1]. It doesn't support CAS on deleteObject.

I don't know about the others though (ABS, GCS, Tigris, MinIO).

https://developers.cloudflare.com/r2/api/s3/extensions/#cond...

[0] https://docs.aws.amazon.com/AmazonS3/latest/API/API_CopyObje...

[1] https://developers.cloudflare.com/r2/api/s3/extensions/#cond...