I'm Micki, and I'm a HubPages staff member, the Head of Product.
We decided to remove the Bookmark feature because it received very little use. We are just starting a series of design updates to HubPages with the goal to make it easier to use and get your readers more engaged. In order to do that, we need to retire features that do not get used much in order focus user attention, minimize distraction, and reduce our engineering maintenance work.
There is a bit more detail on this feature going away in the blog post from Jason, our Head of Marketing, from March 20:
You can still access bookmarks that you have, and we will soon give you a way to download and save your bookmarks before we eliminate the feature entirely.
Hope this answers your question.