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Sometimes deleted labels might be still suggested as recent labels. This is caused by Jira storing recently used labels in browser local storage. There is ticket to fix this in Jira Issue tracker.
In the meantime, you can delete the local storage data from your browser. You can To do that, open Console → Application (or Storage in Firefox) → Local Storage. After that, find your Jira URL, right click and use Delete All popup to delete temporary data stored in your browser.
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