Slack is a team communication tool. That means it has slightly more robust features than a simple chat app, like threading conversations or channels for specific topics.
Slack is not Open Source and it is not end-to-end encrypted.
It was acquired by Salesforce, who have been gradually reducing free options as it orients it towards enterprise companies.
Admins of a Slack could hypothetically read private messages between members of the Slack.
Free Slack now has a 90 day limit on access to messages and their history, requiring you to pay for a paid account to access them. This puts you on a per user tariff, which can be expensive for large community groups.
Fortunately, there are similar apps like Element, Mattermost, Rocket.chat and Revolt.