During our meetup we were not aware or were not able to remember any open source and open standards based team chat software/service, ie. similar to Slack or Stride (formerly HipChat).
However it looks like there may be an open source (not sure if open standard) team chat software/service called Riot (formerly Vector)
We also mentioned some approaching:
- Movim https://movim.eu/ IM and social network, not really a team chat
- Mangosta iOS/Android but it is nowehere near ready, and won't be anytime soon https://github.com/esl/mangosta-ios https://github.com/esl/mangosta-android just code examples
In the non-XMPP open source world there is:
- Mattemost, Rocket.Chat, Let's Chat, Zulip, but specific protocol and not federated
- Ring, based on SIP, but using DHT and Blockchain
In the closed-source, proprietary world, there is:
- Cisco Spark
- Microsoft Teams
- Google Hangouts Chat
- Discord, initially for gamers, but becoming more widely used
As we could observe, the market craze is high about "Team Chat" or "Team Communications", sometimes coined "ChatOps".