So that users can rid themselves of this red message, a quick delete link, for the offending series would be helpful from the System view.
When Sonarr reports that a series was “removed from the TheTVDB” users can then quickly purge that series from their list. The quick link can remove the series from their list, along with an option to exclude from imported lists and deleting that series’ files (if any detected).
Not only would this feature save users time, but it would also cut down on new tickets for that error message, because users can immediately solve the reported issue.