At the very least I would expect Apple, or any email provider, to be able to restore everything in a disaster recovery situation. i.e. return everything to the point of failure. Most businesses keep messages much longer.
At my last job I asked management how long they wanted me to maintain an archive of email messages. With the ability to de-duplicate attachments, etc. disk storage isn’t usually a factor. We chose to keep all internal and external email and attachments for seven years.
Since mail was delivered to both the user and the archive even messages that were immediately deleted from the user account remained in the archive.