I've been in a few slack communities that were on the freemium plan. On that plan, older messages become in-accessible if you don't pay. They don't get deleted of course: you will get them back if you switch to a paid plan. But in practice that seems to be rarely interesting. Chat is all about what happened recently and not about what happened weeks/months ago. Ephemeral messages are a good default.
I'm curious what the legality of that is under the GDPR and other preceding data-protection regs...
If you're using Slack in a personal capacity, then don't you have the legal right to demand an archive of your messages without paying them anything?