Good riddance. The number of production deploys I've seen pointing at bucket names that could've been claimed by anyone was wild. Glad this is finally getting closed off at the platform level instead of relying on everyone to not make the mistake.
There are other mitigations though: You can pass expected owner accountId on S3 operations and you can create SCPs that restrict the ability of roles to write to buckets outside the account. Unless you have an account that does many cross-account S3 writes, the latter is a simple tool to prevent exfiltration. Well, simple assuming that you're already set up with an Organization and can manage SCPs.
There are other mitigations though: You can pass expected owner accountId on S3 operations and you can create SCPs that restrict the ability of roles to write to buckets outside the account. Unless you have an account that does many cross-account S3 writes, the latter is a simple tool to prevent exfiltration. Well, simple assuming that you're already set up with an Organization and can manage SCPs.
[0] https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucket...