While looking at ETag handling, I noticed that if-(un)modified-since headers are completely ignored.

These headers are specified for GetObject, HeadObject, CopyObject, UploadPartCopy APIs.

S3 now also supports conditional writes and conditional reads:
https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-requests.html
https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-writes.html
https://docs.aws.amazon.com/AmazonS3/latest/userguide/conditional-reads.html

S3Mock should support those types of requests as well.