Forgot to mention: at my job, Slack threads have been super useful. Three or four people can discuss one topic in an open channel (so others can join in and we can search history) without spamming notifications against everyone else.

E.g. someone could put a message in #engineering about a bug. All the engineers get that notification so anyone who knows more details can chip in immediately. They then reply in a thread on the original bug message so they can discuss without everyone else getting bothered with irrelevant notifs AND without locking the conversation into a group DM where nobody else can chime in nor can we all easily search for it later if it’s relevant.