Considering a Slack replacement for an open-source organisation. Here's the current contender list, in preference order, without much thought but based on personal experience. Got any other suggestions? :)
1 - IRC
2 - Mattermost
3 - RocketChat
4 - NextCloud Talk
105 - Signal
400 - Matrix
9000 - Discord
9998 - Skype
9999 - Slack
10000 - Zoom
10001 - WebEx
@popey used RocketChat on a vps for 5 years with minimal upkeep. Now hosting it on prem on a cheap microserver and still running well. Great slack alternative with channels. Files can be hosted externally (inc on Nextcloud) Mattermost might be a good contender too?