You need to establish some rules to make Slack work for most teams without overload.

1) People should avoid DMing each other when talking about work, use threads instead (see 3) to discuss topics in team channels (see 2).

2) Keep channels to a minimum. Every person shouldn't have more than 2/3 channels they need to pay attention to. A private team communication channel (eg #dream-team) and a private work channel (eg #dream-team-engineering) is enough for most teams. All team members are in the private team communication channel. A work channel is specific to a domain, such as engineering. Example you don't want engineering work banter to distract those working on other stuff.

3) Use threads religiously! This is super important as it helps declutter the Slack experience and keep conversations heavily organized.