Hi. Actually if you delete a banner (clicking the "delete" here for example, the system will only delete the MediaWiki page where the banner code is stored, leaving undeleted other MediaWiki pages, CNBanners and translations. Those should be gone too when deleting the banner as no longer needed.
In the past I remember an "Automatically deleted by CentralNotice" message in the deletion log.
For example, I deleted today this banner through the "delete" button, but related pages remains.
- MediaWiki:Centralnotice-WMAR NatRes-text1
- CNBanner:WMAR NatRes-text1/es
- CNBanner:WMAR NatRes-text1/en