coldlist.net is a small, self-hosted Matrix homeserver for a closed technical circle — isolated, end-to-end encrypted, and run for the people who already have a key.
Any Matrix client works — Element, FluffyChat, Cinny, nheko. Use a custom homeserver and enter:
Accounts on this server look like the line below. This homeserver runs isolated — you can talk to everyone here, but not across other servers.