Engineers use the term.
So do mathematicians and data scientists.
Even journalists like myself qualify.
Anyone who "thinks outside of the box" and has to deal with complex ideas, usually sitting behind a computer or laptop and pondering data sets, analyzing results, and wading through research to find information and explain it to others--that's what we call a knowledge worker.
The term, invented by Peter Drucker way back in 1959, has served us well. At the same time, it's also worn out its welcome.
We now know that robotic helpers will take over some menial tasks, and possibly even work on your behalf so you can retire. Eventually, we'll all qualify as knowledge workers--and in some ways, just about every job under the sun has some element of computing and "knowledge work" involved. We're all knowledge workers to some extent.
To me, it's a bit like saying you are a computer user, or a typist, or an Internet guru. Over time, phrases that describe what we do lose their meaning because we all use computers, we all type, and we all browse the Internet. Just when you think there isn't any element of knowledge work in a job or that a computer has no purpose you find out--of course it does. Every person who works has knowledge of their craft and has to interpret data for others.
I learned this once when I volunteered to help a cattle farmer who was diagnosed with multiple sclerosis. I thought I was going to work in the mud, maybe throw some feed. I was wrong. The first thing we did was on a computer, mapping out the area where we would work, and then we started analyzing some of the medical history he kept for the cattle. Eventually, we did head out into the field, but the job was surprisingly technical and not as "menial" or rough as I suspected. He was part veterinarian, part scientist.
Computing is so pervasive now. It helps janitorial staff plan out their cleaning cycles. It's a major aid for those who work in the arts, especially when it comes to scheduling and image editing. Construction jobs rely on tacit knowledge and complex data sets as well.
The term is almost meaningless. We might as well say worker.
So what should we say instead?
My vote is to stop saying knowledge worker at all, and to use the title for the job people have, to stop trying to categorize workers when it doesn't really help. If you're a graphic designer, or an engineer, or a journalist--congratulations, you're also a knowledge worker. The category has lost all meaning, so it creates even more confusion. Strictly defined, if a knowledge worker is someone who works with knowledge, we all qualify.
We should also resist the temptation to come up with terms that don't really help quantify what we do. Originally, way back in the 70s and 80s, it made sense to say knowledge worker because not everyone analyzed or processed information as the main part of their job. And, not everyone typed on a computer or had access to the Internet.
Today, we know better.