I definitely think bookmarking in Wikipedia should not only be kept but further improved. Customisation by topics should be allowed, something like a traditional folder hierarchy or normal browser bookmarks organisation. Hard to understand Collections feature cancellation.
Topic on Extension talk:Gather
Bookmarking is important
Thanks for your comment @OneFingerSnap.
If you want to learn more about the cancellation, there is more information and comments on the RFC.
The feature needs more work to be considered useful and it has rough edges to improve, but it doesn't have the proper support at the moment. Thanks for stating your opinion!