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It is not possible to hide the checklist panel in Old Issue View. It is Jira's limitation.
New Issue View
New Issue View can be enabled and disabled on the user's Settings page.
- Make sure "Show Checklist Always" option is disabled in Global Settings.
- Remove all checklist items (manually or using the editor).
Hide checklist panel using the "..." menu
Info Please note that the "..." menu button is located in the same line as the main bold title "Issue Checklist". Below (not visible on the above screenshot), there is another and larger "..." menu button that provides checklists and templates actions.