You can also remove Slack channels as needed. A popup will appear if you try to delete a Slack channel that is associated with a preexisting alert. In this case, click the link to go to the corresponding Edit Alert page. Next, update the channel to an active one before completing the deletion.
Similarly, the “When an issue is reopened” message will appear if the Slack channel you want to delete is the assigned one for reopened issues. It will link to the Settings page of any apps in the organization where this is the case. Click through and update the channels where needed before continuing with the deletion.