In a way there's a second single point of failure though, right? So many people use Slack to integrate all kinds of things, and rely on their interaction with those platforms through Slack, that if Slack goes down then productivity halts and it's totally out of your hands while Slack themselves try to resolve the issue.
- You don't get GitHub notifications on pull requests and comments, so things don't get reviewed and merged if developers aren't in the habit of checking the PR tab on GitHub itself.
- You don't get CI notifications so you won't know how your latest test run or deploy is going without going straight into the CI service itself. Even worse when there's a failure and you're too used to having Slack warn you about that.
- Your team might depend on Slack so much that they don't know how else to efficiently communicate, and the most efficient channel to communicate a fallback is not available or rarely checked (e.g. email, face to face). So you get a lot of chaos as people come up with dozens of alternatives.
This is just poor discipline more than anything, putting too many structural eggs into one basket, but it doesn't change the fact that Slack has created that dependency.