Since this problem is with the GitHub API I suggest you could retry with backoff logic, but when GitHub has more serious outages that last for hours, like on May 22nd, you could move the task to a retry queue that would retry until the GitHub API is healthy again, or until the notification is no longer relevant.
If you could quickly implement an option to manually re-trigger the notification that would be great.
Alternatively if there is a way for us to ‘fake’ the notification from our side, we could probably write a script that would be given a PR number and notify GitHub that Sonar had completed the analysis. We can retrieve the analysis url from the SonarRunner output and follow breadcrumbs I guess.