Makes sense to be able to delete unused custom fields, even if it has to be through a support ticket. And it would also be desirable to be able to edit API names for custom fields. Sometimes it's simply not enough with hiding or renaming the field but being unable to edit the API name.
We still need this feature. I'd like to be able to delete them even if they have data. This would allow us to get rid of fields when it is two years later and the custom fields are no longer relevant because a campaign was finished a long time ago.