The same thing is true of of the Lincoln speech, in a somewhat different way.
Does anyone really need to be warned that they are using the passive voice? Does anyone actually do that by accident?
Yes, all the time. I write a lot and still fall into the trap, and it is hard not to do it sometimes especially if you were only introduced to the concept later in life. I try hard not to use the passive voice, but there are times when I think it is acceptable, and there are occasions where the alternative is more confusing. The quoted speech "Now we are engaged in a great civil war..." sounds perfectly fine to me!
Honestly, I have trouble thinking of an alternative sometimes, even when I can identify a passive voice sentence.
I see it quite often in business writing, I think people fall into it because it's a way to avoid taking responsibility. "It has been decided" vs. "I have decided." I would not say that it's always intentional.