Mattermost
Mattermost is an open-core, self-hostable online chat platform that provides secure team messaging, file sharing, search, and integrations with third-party applications. It is designed as an internal chat solution for organizations and companies, positioning itself as an open-source alternative to Slack and Microsoft Teams.Note: at Cloud68.co we do NOT develop this software and all the names and trademarks of the software are owned by the entity behind the project.

Data retention
On Mattermost, data retention unfortunately is only possible on the Enterprise and Enterprise Advanced Plans. Here is the source of the information  https://docs.mattermost.com/administration-guide/comply/data-retention-policy.html .