AFAIK that occurs here without being focused (limited) to those topics. The name Hacker News even implies that was the focus here at some point. Beyond HN there is StackExchange [1] but they don't really have discussions. They are more Q&A. TheRegister [2] also has a forum and they will discuss Security topics, but they are not limited to that topic and the discussions are less thought out, more emotion driven and reactionary. There are IRC and Discord channels/servers that are security focused that may or may not match your criteria.
[1] - https://security.stackexchange.com/
[2] - https://www.theregister.com/