Ceph has strong consistency by design. S3 compatibility in Ceph comes via the Rados Gateway (RGW) which just sits atop normal Ceph. This means that the strong consistency is carried over to RGW. So, no change for Ceph.

OpenStack Swift has an eventually consistent design. If the Swift community desires to maintain strict compatibility with S3 regarding consistency model, it would be a challenge. Despite being a challenge, I would never bet against the Swift developers. If they want to do it, I'm quite sure that they can. Having said all of that, I doubt that the Swift community would consider this a burning issue that needs to be solved. They've existed for over 10 years with an eventually consistent model and their user base is well aware of it.