User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment-4130826-20150518202641/@comment-1038387-20150518211626

It wasn't a glitch. They wanted to remove it in the long run, but accidentally removed it earlier than planned. That was mentioned in the comments on this blog.

It's not an optional feature, it's gone for good. There are some people that created a custom module, but you can only use that in personal CSS, not site CSS.