We explicitly stopped using slack:
from: [ros-users] ROS Community Discourse
After feedback about the failings of Google Groups and the non-open,
proprietary nature of Slack, we launched an experiment migrating the ROS
Java SIG from a Google Groups mailing list to Discourse.
…
Slack is officially deprecated: you are free to continue using it, but if
you want something to come onto the radar of the ROS team, you should
prefer to use Discourse, IRC, Github, or email.
So in some since this topic is a rehash of discussions we had then, but notably at that time we didn’t really consider discord or rocketchat (AFAIR).
One big issue was the ability to get access to historical chat records publicly via a website or some rest API, and the fact that Slack limits chat backlogs if you don’t pay for it (not sure if that’s still the case).
It’s a valid concern about the other options like discord (it has chat logs available via a rest API, but no convenient export feature) and rocketchat (presumably not an issue since it can be self-hosted).