As we said in May, one of the reasons Google removed it was due to user confusion. Google wrote, "users were often confused about the keywords listed in content keywords."
Google said now that there is Fetch as Google and Search Analytics, seeing how Google crawls your site is easier. When this first launched, it was the "only way to see what Googlebot found when it crawled a website," Google said.
So Google is dropping it.
Marie Haynes said:
I liked Content Keywords. It was a great way IMO to look for hacks and also to determine what Google thought your site was about. https://t.co/wGZWcbOWwK— Marie Haynes (@Marie_Haynes) November 29, 2016
True, my original complaint about dropping it in May but Google does send nice hack warnings.
Google also recently dropped the demote sitelinks feature as well.
Forum discussion at Twitter.