A crosswalk button that does nothing is a placebo even if that wasn't the original intent.
The elevator button is more complicated but I think the typical uselessness combined with misleading label is enough to qualify. The reason it exists isn't placebo, but the way it's set up makes it a placebo, even if it's via negligence.
The term "placebo" implies intent. So saying something is "a placebo even if that wasn't the original intent" is like saying something "is red even if it isn't that color."
The elevator button is more complicated but I think the typical uselessness combined with misleading label is enough to qualify. The reason it exists isn't placebo, but the way it's set up makes it a placebo, even if it's via negligence.