Does deleting a question bank delete all of the questions in it?
Deleting a bank will delete all of the questions that are in the bank but it doesn't necessarily remove all occurrences of the question.
Deleting a bank can be problematic if the question bank was used linked to a quiz. When this happens, the questions become embedded in the quiz directly like they had been found and included. I'm not sure exactly how that works as I only tested it with a question bank with a single question, but that single question became part of the quiz when I went back to edit it. That did not come through as a question group, but that might have been because it was just a single question.
Any students who have already taken a quiz, whether the questions in the question bank were linked to the bank or found and included as part of the quiz, do not lose their quiz results.
There is a trick for undeleting material that you accidentally delete. Question Banks are not included as part of this. If you have accidentally deleted a question bank, you will likely need to reach out to Canvas Support to recover it. If you accidentally deleted it since Saturday, you may be able to go into your beta instance of Canvas and pull up the question bank and recover it to a file (not so simple process, so I'll just mention it as an option.)
Retrieving data ...