It's been a feature request for ages. I strongly suspect we're seeing it land now because it was legitimately that hard to implement it atop the existing search infrastructure.
You're right about the risk of filter bubbles, but the door swings both ways; this is the feature people have wanted to be able to say "I want to look up this JavaScript command, *and I want no fucking links from w3schools thankyouverymuch*" since w3schools was called expertsexchange.
(In terms of the filter bubble risk: oh yeah, that's a good question. *Sure would be cool if Google had some kind of 'ombudsman' position to weigh in on these kinds of questions, wouldn't it?*)