I have used Delete and Discard, but I can't find these issue now. Where are they?

If issues were deleted and discarded, you can find and undiscard them from the inbound filters page. Navigate to your Project settings and under Inbound Filters you'll find your discarded issues:
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Please note, removing an issue from the discard list does not recover events for that issue. It will only allow ingestion for events of that issue again.