S3 is almost certainly not fully partition tolerant at the node level and requires some sort of quorum. Other “magical” data stores like Spanner also retain this limitation, they just have very reliable replication strategies.
Whoa hang on! You can't just say you're not tolerant of partitions...
I'm struggling to find the best post on aphyr.com about this but https://aphyr.com/posts/325-comments-on-you-do-it-to is a good one, specifically the line:
"CP and AP are upper bounds: systems can provide C or A during a partition, but might provide neither."
In short, Partitions happen, no matter what. No really, when you have multiple nodes they happen, you must tolerate them. You can't have CA.
The answer in this case is that availability is sacrificed, unless Amazon is making a very misleading claim of strong consistency (per the submission title/link). So it’s CP.
In case “not fully partition tolerant” is what is throwing you off, all I mean by that is that there are likely network partitions of the S3 nodes that will not cause outward failure (e.g. partition of a single node), though some will (e.g. too many partitions to form a quorum, or whatever their underlying consensus implementation relies on).
So they claim performance and availability will remain same while claiming strong consistency. I was confused at first but then “same” availability isn’t 100% availability. So it indeed CP.
I guess the simple answer to the GP is "Availability is likely sacrificed", and the clue's in the submission title being all about consistency. I haven't looked too deep into it and am sure there are also still Consistency sacrifices too.
I'm still a little niggled though... "almost certainly not fully partition tolerant at the node level and requires some sort of quorum" - what does this actually mean? I'm confused why you raise the tolerance of a single node to partitions, when the partitions by definition require >1 node?