Slack is published by Salesforce. This page compares only projects with an explicit relationship to it; shared categories alone never create an alternative claim.
Recommended order uses OpenAlternative organic alternatives v4 (version 4). Relationship confidence, category fit, logarithmic popularity, as-of maintenance and freshness, bounded anomaly quarantine, and known-data penalties. Payment is fixed at zero.
A hard constraint removes a Project only when its stored fact is known and conflicts. Unknown facts remain visible, are called out below, and never become a negative claim.
Team messaging for organisations that cannot use hosted chat
Mattermost is channel-based team chat designed for environments where the message history has to stay inside a boundary: defence, healthcare, and regulated finance are its natural constituency.