Slack Alternatives

I saw no mention of these Slack alternatives (all Open-Source):

* Matrix (https://matrix.org)

* Mastodon (https://joinmastodon.org/)

Both could be put to use as a Slack alternative, but lack a number of the bells and whistles that make Slack an easy experience.

Mastodon in particular I could see replacing Slack, if it became more polished and they made an organization-centric version/mode of their UI.