One message, the right channels
In the web app, open Broadcasts and choose New broadcast. Write the message with Slack formatting, bold, italic, code and links. The Preview in Slack shows the text as Thorpe will post it, before the formatting is applied.
For the Audience, choose All customer channels, By tier for only the channels mapped to an account with that tier (Enterprise, Growth or Standard), or Pick channels to tick them one by one.
The line under the preview says how many channels will get the message and names them, so you know the reach before you send. Customer channels that have been disconnected are left out, and the line says how many.