Not a full replacement but there is Garage, which was quite well received in other HN threads. https://git.deuxfleurs.fr/Deuxfleurs/garage
Afaik Ceph has its own object-storage functionality as well, which seems to be S3-compatible: https://docs.ceph.com/en/latest/radosgw/#object-gateway
This is a set of unofficial Amazon AWS S3 compatibility tests, that can be useful to people implementing software that exposes an S3-like API. The tests use the Boto2 and Boto3 libraries.
https://github.com/ceph/s3-tests