Issue channels are channels that have been created when a user is viewing a JIRA issue and chooses Add Channel in the right panel under Mattermost settings.

See Creating a Mattermost channel from an issue in JIRA.

Here in the Mattermost administration it will list all such channels that have been created via a JIRA issue.  The name of the channel is the key of the JIRA issue. 

Click to Disable an issue channel.

Click to Enable an issue channel.

To delete an issue channel click .