The "highly active" term has now replaced the "protected" term, but the principle is the same: these are questions that have been specifically marked by moderators to prevent new users with very low reputation to answer.
It is typically used for questions that get viral (due to the Hot Network Question list, or some tweet - hence "highly active"), so that people that aren't quite aware of the site rules don't pile in just to answer things like "great question", or "I experience this too", or some other kind of noise or spam.
Recently, the post notices for closed/protected/locked/deleted posts have been refactored by the SE team. So now, the "protected" term has been deprecated in favor of "highly active". It is possible that these questions aren't actually highly active anymore, but at some point, they have been. And they are still in that state where low rep users can't answer.
There is more details about the significance of this statuts on the main meta: What is a “protected” or "highly active" question?
Also, see this post about the refactoring of post notices: New Post Notices are live network-wide. And have a look at pkamb's answer there, which summarizes pretty well how this new "highly active" notice is, indeed, very misleading.