The project here is not under an open source license as defined by the 10 criteria of the Open Source Initiative. There is the only acceptable definition of what is open source today, and you can refer to it by clicking on this hyperlink: https://opensource.org/osd
The publication will be deleted as out of topic.
I would have said in fact Matrix or XMPP-based solutions but it seems you already have spotted them. Maybe Mattermost?