Every Slack workspace has a general channel (named either #general or #all-companyname), which is the only channel that members are automatically added to and unable to leave. This channel works best when it’s used to share announcements with members of your workspace. 

Restrict who can post

By default, all members have permission to post in the general channel. Owners and admins can restrict who can post, making it read-only for those who don't have posting permission. This helps larger channels stay on topic and free from distraction.

Rename the channel

Owners and admins can rename the channel to something more descriptive (like #announcements or #notices) and set a channel topic and description so that members know the channel's intended use.

Notify your workspace

For urgent announcements, people with permission to post in the general channel can include @everyone in their message. This will trigger a notification for all members of your workspace so that no one misses important updates.