You're reading the public-facing archive of the Category Theory Zulip server.
To join the server you need an invite. Anybody can get an invite by contacting Matteo Capucci at name dot surname at gmail dot com.
For all things related to this archive refer to the same person.
Is there any way of accessing older messages from the CT mailist, which were stored at gmane, e.g. this one?
@Simon Burton has an archive of emails on GitHub.