While I wouldn't publicly hand out my account IDs as a general practice, I think you have to expect that some of them will be disclosed at some point. As more third party vendors and SaaS platforms move away from IAM users and access keys to using role assumption as the preferred method of integration (as they should!), the account ID of at least the account you use as their integration point is now known by another party, who have their own dependencies, vulnerabilities, etc.