The collection feature has not been removed and is available in the normal search and also in the XML and json API of YaCy.
Just go to Portal Design → Search Page Layout → look at the screenshot:
You can add additional navigator fields and there is also the collection field. Just click on the +.
Next, you find the collection in the search result. If you click on the “API” button (upper right) you get an xml (essentially rss) of the result. If you furthermore replace the .rss extension with .json you get the same as json. Please watch how the search query is constructed with the collection attribute, you can do the same with an automated URL construction.