They wouldn't need to store so many if they actually let people delete their messages on account deletion. Instead, they ban many people who attempt to do so via automated scripts.
Deletion of data at scale is a really difficult technical problem, unfortunately.
I'm not saying they shouldn't do that though - especially given regulations like GDPR. Designing systems for deletion is important! But it's also really hard, especially if you didn't design for it from the start.
There's also no way the tiny fraction of users who want to delete their data would make up a significant enough proportion of the messages that it would impact their scaling strategy.
One of the big reasons I refuse to use Discord. Deleting your messages is a right every user should have. Whether it be individually or in bulk. The way it's done now just makes it more susceptible for users to be open to malicious attacks. Whether someone archives your content before you delete it, that's not of importance, that can happen on any internet medium.
