There's a whole FOSS ecosystem of chat/collaboration applications, like Mattermost and Zulip; there's Matrix for a federated solution, and tried-and-true option

There's a whole FOSS ecosystem of chat/collaboration applications, like Mattermost and Zulip; there's Matrix for a federated solution, and tried-and-true options like IRC. For something called "Reclaim the Stack" to lock discussion into someone else's proprietary walled garden is quite ironic.