When you delete a level, Revit also deletes associated views and model elements that are hosted by the level (for example: doors, furniture, and rooms).
A warning message displays so you can see the full list of items to be deleted with the selected level. You can cancel the deletion request if needed.
If you want to delete the level but not its associated views or hosted elements, export the details of the warning list. Using that information, locate each item (use the Select by ID tool), and change its associated level. Then delete the level.
If the level to be deleted does not have any associated views or hosted elements, no warning is issued, and the level is deleted immediately.