Your service overwrites reports/today.json in S3 and the PUT succeeds. Immediately afterwards another process GETs the same key. What does it receive?

It receives the new version. Since December 2020, S3 gives strong read-after-write consistency. This means a read right after a successful write always sees that write. It applies to PUTs and DELETEs of objects, and to list operations.

"Possibly the old version for a few seconds, because S3 is eventually consistent for overwrites" is outdated information that people still repeat.

One caveat: if two writers write the same key at the same time, the last write wins.