One of the least-touched courses I know of is Crystal Downs. Because it was a summer club for the membership, they didn't want to take it out of play with silly revisions, and they didn't want to spend a lot of money on it, so very little construction was ever done. It's one of the only courses I've seen where the bunkers shrunk over time [grass growing down into them] instead of expanded [due to edging].
But I guess it depends what you mean by "left alone". In spite of the above, someone planted a bunch of trees in the 1960's, and it took until a few years ago to remove them all. Also, heavy sand topdressing on the mucky greens has built up the greens relative to the surrounds during the time I've been around the course, and where the green had shrunk a bit before the topdressing program started, hole locations were lost. [We have started to do work on a couple of these areas this fall ... there are a lot more to do.] Further, the nature of the roughs has changed completely from 30 years ago due to new irrigation and more management; I am trying to steer it back to how the course used to play.
So, even leaving a course alone is not enough, it is always trying to change in subtle ways. Still, to me, it's better to leave it alone as long as you can, than let different architects tinker with it. I have seen some bad examples of the latter in the past week.