Deleting a list will delete all data connected to that list including all email campaigns sent to that list. You are warned about this before deleting. There is no way to delete a list and still retain the campaigns and statistics connected to that list. If you would like to tidy up your account you can instead archive lists you are no-longer using. When viewing your list of lists, click on the gear icon to the far right of the list and then click on the Archive button.
If you have a large number of lists this is often because you are importing a new list for every campaign you send, we consider this bad practice as it may result in continuing to send to people who have unsubscribed form your previous lists. It is better to update and maintain your list so that unsubscribe data is not lost. If you must upload a new list each time then please make sure you are removing unsubscribes from your lists before importing, please see Managing your lists outside of BoltMail