Jitsi isn't really a Slack alternative. Instead, it's more of a Zoom alternative.

However, Matrix is a great Slack alternative. Slack channels are similar to Matrix rooms, which can be organized into Matrix spaces. Matrix supports threads, replies, attachments, and formatted text like markdown or HTML. Slack's snippet functionality is not as great on Matrix and Slack's integrations with other services are likely easier to setup. There is likely a bunch of other pros/cons to Slack/Matrix depending on your use cases. The caveat is that you'll need to use a Matrix client and Matrix homeserver that support the Matrix functionality that you want.