Deleting subcollections removes items from branch
I've been trying to tidy up the over-complicated collection tree that I've built up over a long time. I was surprised to work out that deleting a collection (but not its items) removes the item from the branch of the collections; it remains in the library and other collections, but it is removed from the parent of the deleted collection. This happens whether extensions.zotero.recursiveCollections is set to true or false. Perhaps this is obvious to everyone, but it does make simplifying the collection structure much more difficult.
Upgrade Storage
If you create top-level collection A, subcollection B, add an item to both collections, and then move subcollection B to the trash, the item remains in collection A.
(Also, there's no reason to refer to recursiveCollections anymore. It's just been View → Show Items from Subcollections for years.)
If you actually added it to both collections, the item would remain after deleting the subcollection.