As of November 2024, you can append to objects in S3, but only in S3 express one zone buckets. Note that this means less cross-AZ replication, and pricing that is about 8x as high as regular buckets. You do get substantially faster performance and lower latency though.

The announcement: https://aws.amazon.com/about-aws/whats-new/2024/11/amazon-s3-express-one-zone-append-data-object/

The documentation on the API: https://docs.aws.amazon.com/AmazonS3/latest/userguide/directory-buckets-objects-append.html