Matrix.org

Matrix is an open standard for decentralised realtime communication; the organisation behind it, Matrix.org, is based in the United Kingdom, which means Europe but not the EU. The protocol covers chat, internet telephony and video conferencing. Users with accounts at different providers can talk to each other, much like with email. Those who wish can run their own server and thereby keep control over their data; end-to-end encryption protects the content. Suitable programs and mobile apps are available for download. Listed alternatives are Discord, Signal, Slack, WhatsApp and Zulip.