A way to find all “missing entries” ? using UI or JS API ?
This happens when the referenced entry (most likely a Promo Banner Section entry in your case) is deleted, but the reference or link to that entry still exists in the another entry. The Contentful webapp will warn you if you try to delete an entry to which other entries refer, like so:
You also will be blocked from publishing updates to the parent entry when it contains a non-resolvable link like you mentioned.
Can you provide some more details about your situation and what exactly you are trying to do?
Thanks!