What do you use if not slack? OPs advice is standard best practice. Respect peoples time by not expecting immediate response, and use team or function based channels as much as possible.
Other options are email of course, and what, teams for instant messages?
The bullet points for using Slack basically describe email (and distribution lists).
It’s funny how we get an instant messaging platform and derive best practices that try to emulate a previous technology.
Btw, email is pretty instant.
I’ve always that forums are much better suited to corporate communications than email or chat.
Organized by topics, must be threaded, and default to asynchronous communications. You can still opt in to notifications, and history is well organized and preserved.