I personally believe that the proliferation of infinite scrolling/pagination has a strong correlation with the disappearance of the scrollbar.
In a world before infinite scroll, the bottom of the page was indeed the last bit of content on the page. In today's world, scrolling to the bottom of a page typically loads more content.
In this new world, I can imagine designers and engineers deciding to do the simple thing which is to remove the scroll bar since it's no longer usually an accurate depiction of where the users position is in relation to all the content available.
Of course all of this solely applies to feeds and not standalone pieces of content like articles. In the latter cases I'm unsure beyond minimal aesthetics why the scroll bar would be removed.