tl;dr You can read about our API strategy here and see the roadmap here.
There are no current plans for Bookmarks to come to the API - as we stated on the blog nearly a year ago, the API plans are focused on data features and access, more than on delivering client app product features. The roadmap is public and you can follow where we are focusing our attention there. Thanks.
(while we’re discussing the roadmap, I’ll also take this opportunity to remind you that the legacy Direct Message APIs, along with User Streams and Site Streams, are due to be retired on June 19, 2018, so please migrate any code that depends on those endpoints before that time)