Firecracker doesn't reclaim RAM well, so they put this limit in to make sure you don't suffer from the eventual memory bloat problems.
"NoncurrentVersionExpiration": {
"NoncurrentDays": 30,
"NewerNoncurrentVersions": 3
}
That way when someone screws up and your AWS account gets owned, or your databases get deleted by an agent, it doesn't have enough access to delete your backups, and by default, even if you have backups that you want to intentionally delete, you have 30 days to change your mind.