Is there a way to not change an S3 object's Last-Modified date when copying it to a new bucket?

Hello,

You can use S3 Replication feature to retain S3 object’s Last-Modified date when copying to a new S3 bucket where S3 Replication retains all metadata, such as the original object creation time and version IDs. For more details, please refer: Replicating objects - https://docs.aws.amazon.com/AmazonS3/latest/userguide/replication.html

Amazon S3 automatically updates the last-modified field for object whenever it is created, updated or replaced. You should be able to see it in the S3 console or by using CLI (head-object).

Thank you!