Seems like very basic mistakes were made, not at the event but way long before. If you don't test to restore your backups, you don't have a backup. How does it go unnoticed that S3 backups don't work for so long?
Virtually the only way to lose data is to not have backups. We live in such fancy times that there's no reason to ever lose data that you care about.
Best way to ensure that is to have backup restoration be a regularly scheduled event. For most apps I work on, that's either daily or (worst case) weekly, with prod being entirely rebuilt in a lower environment. Works great for creating a test lane too!
My uneducated guess (this one hit a friend of mine): expired/revoked AWS credentials combined with a backup script that doesn't exit(1) on failure and just writes the exception trace to stderr.
I've seen that many more times than I've seen the "no alert" option.
Existing team: "Yah don't worry about that. It does that every day. We'll get to it sometime soon."