What I always miss in these lists is an alternative to Slack, which I also think would be one of the most important to get right since:

• Slack is used in companies and to transfer operational information, therefore contains both HR (personal) data and sensitive business data
• Slack generates a lot of data now used to train LLMs and other ML applications
• Slack is seen as a must-have in most companies and it gets in at an affordable price point which still could be lowered by an EU solution.

I have looked into Whaller, but would still prefer an EU-made open-source version of something like Mattermost.