AHA!
They are groups of multiple objects that were added to the library as one item.
Normally, if you select multiple objects and drag them to the library, they all get added.
If you hold Ctrl as you drag, they get added as one item with the + (with a very boring name like "Copy of 2 data blocks".)
If you drag one of those DB+ items into your project, you'll get multiple DBs in one go. It looks like each of the groups/folders in the old library got combined into one item.