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.
We use self-hosted Mattermost (team version, i.e. without limits but no enterprise features like LDAP). Fine for a small team (around 40 active users here) where you can script account actions via the API, probably not fine when users become a lot more, or you might need access to the compliance functions for audit purposes, etc.
For us the free version of Slack was insufficient, the commercial one too expensive, and anyway, given that it's a cloud-based system, it's not compliant with our internal rules for confidential information (unless we can get some specific agreement with them). On the side, there is a bit too much analytics/telemetry in the Slack client.
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.