Alessandro Alpi shows how to integrate SQL Server notifications with Slack:
Now, how can we send notifications from SQL Server in an easier way than using custom code or a Slack incoming webhook? Is there any integration or a Slack app? Yes. And guess what? I think you’ll like it because you don’t need to write a single line of code, and you don’t need to choose between CLR, PowerShell or any other language. It’s ironic, but the integration is called “Email”.
Speaking of CLR, I’ve had success with the SqlServerSlackAPI in the past.