I haven't found one of the many directories (https://www.goeuropean.org/, https://european-alternatives.eu/, etc.) that offer an API to this data. More than listing alternatives, for real adoption, we need to put this information in front of regular users when they need to use/pick one of the non-EU products/services.
A Matrix server is a good solution. You can disable federation if you would rather not talk to other instances and have a similar experience to Mattermost, Slack, etc.
If you dislike the Matrix protocol, an XMPP server would work exactly the same way.
NextCloud Chat could also be a viable solution for internal chat.
Does this site offer an API?
I haven't found one of the many directories (https://www.goeuropean.org/, https://european-alternatives.eu/, etc.) that offer an API to this data. More than listing alternatives, for real adoption, we need to put this information in front of regular users when they need to use/pick one of the non-EU products/services.