Slack said they’ll start deleting old data from free workspaces. While I understand why people are frustrated, you can’t trust your data to someone else’s server. If it’s important to you, keep a copy somewhere under your control.
It might be inconvenient to self-host something like Mattermost, but perhaps not as much as having a company delete all your data. https://slack.com/intl/en-gb/help/articles/29414264463635-Updates-to-message-and-file-history-on-free-workspaces