If I get it right, from all the group messengers that appeared around 6 years ago, the only one with an open protocol seems to be Matrix, but it hasn't proven to be to their advantage, apparently. Only Element, Zulip and Mattermost are open source?
Honorable mention: IRCCloud was a "hot thing" for a while, but it seems like it wasn't enough to revitalize IRC. [2]
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Appendix, initial releases:
* Slack: August 2013; 7 years ago
* Discord: May 13, 2015; 5 years ago
* Mattermost: October 2, 2015; 5 years ago
* Zulip: ca 2014, at least 5/6 years ago?
* Matrix protocol: ca September 2014; 6 years ago
* vector/riot/element: ca September 2016, 4 years ago
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1: https://techcrunch.com/2020/09/30/element-acquires-gitter-to...