It seems safe to assume that it varies by family and locality.
I have found a number of people within my DNA matches, and have helped many find out how they fit into my tree. With that being said, it's pretty uncommon with my relatives, and I'd say many of the DNA matches are in the generation after me.
My closest such biological relative is a daughter of one of my 2nd cousins, who apparently got a girl pregnant in college in the '60s.
On my AncestryDNA, my 1st mystery relative is my 19th match, 153cM. From the shared matches he must be on my gt-grandfather Schmitz's side, so probably a 3C. He didn't reply to my query about it. He almost has to have an NPE situation in his tree somewhere.
My 26th match, at 123cM, is the aforementioned 2C1R.
My 27th match, at 121cM, is a son of a woman who had been adopted out. I was able to figure out that she had been fathered by my great-grandmother's brother (a Mahaney), who was a fireman. He went to another city at around that time, got married to somebody other than the biological mother, and returned. Probably his departure was due to his having learned of the pregnancy. So that match is also a 2C1R, but the adoptee was his mom, who is a 1C2R (so I guess she's really my closest such relative). That was in 1898.
My 30th match, at 115cM is another mystery relative. She has to be on my gt-grandfather Baxter's side, but didn't respond to my query. This is not necessarily an adoption case.
Those 4 are the only cases within my top 50 matches (60cM+) that either involve an adoptee, or are unresolved mysteries. Everybody else I know how they're related (except a few, where I have a pretty good idea who they are but am not 100% sure).
One unusual case I had was of a woman who was the product of a one-night stand, and her mother didn't remember who the guy was. So she wasn't actually an adoptee. It turned out her grandmother (a 3C of mine) was tested, so that was easy to figure out. The grandmother, however, was an adoptee - she was the product of an affair that happened while the husband was away fighting WWII.
I can also think of at least 4 cases where a 3C put up a child for adoption. I have a lot of 3Cs.
As far as history goes, my matches seem to match my family tree, going back to gt-gt-gts and beyond, so there don't seem to be any adoptees in my ancestry. If NPEs were commonplace, there would surely be some.