Currently, this plugin is implemented for direct push, i.e. the site admin configures one channel which is used for notifications when ever this channel is applicable.
In addition to that, user should be able to get notified on Mattermost too. For that, they need to be able to configure their personal credentials and channel information.
Postponed
2.3
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.