Google Scholar recently was completely redesigned to match the new modern Google look.
Part of that means some features have changed, were removed or are hard to find.
One complaint in the Google Web Search Forums is about a key feature being removed.
In the old advanced search settings you were able to limit or filter your search by "search only articles in the following subject areas" but that feature seems completely gone in the new interface.
Here are screen shots:
Google has yet to confirm if it is completely gone, coming back soon or hidden somewhere where no one can find it.
Forum discussion at Google Web Search Forums.