Currently, if you have a collection created entirely either in the new/legacy (CMS) Collections admin, deleting it via UI won't reindex the products in it, consequently not effectively removing them from the collection.

1. Create a collection using the new admin and insert products to it;
2. Check that the added products will be reindexed and their indexed info will contain the collection's data;
3. Click to delete the entire collection on the main UI
4. The indexed info data of the products contained previously within this given collection should have been updated.

Use either the Legacy (CMS) collections module for deletions or the collections APIs: https://developers.vtex.com/vtex-rest-api/reference/catalog-api-collection