Update as of February 2025:
Mattermost has introduced user limits to the free version as part of an ongoing process to better align it with its intended use for small teams. This has been implemented gradually, with admin warnings and hard limits. Currently, the limits are set at 5,000 users (warning) and 11,000 users (hard limit), and we will continue lowering these thresholds until reaching 1,000 users.
For more details on these changes and the reasoning behind them, you can refer to this discussion thread where Ian has provided additional context: Feedback on Collaboration for Mission-Critical Work.
Let us know if you have any questions!