No, I don't think manufacturers are installing and deceptively advertising a 'placebo button'; if nothing else they'd risk doubtless reputation and revenue-damaging exposure at the hands of enthusiast publications which evaluate such things as a matter of course (likely promptly followed by the pesky/costly 'class-action industry').
There are of course notorious cases of auto-manufacturer deception, but IMO (YMMV) this isn't an arena where that's likely to be prevalent.
Methinks the 'terrain selector' and similar buttons are bona-fide, making (perhaps subtle) changes in how the the default 'traction management' is employed in many cases. I've read independent white papers describing this in some interesting detail relating to my Honda Ridgeline,
Wikipedia's Gen 2 Ridgeline article discusses it a bit, including a graphic to illustrate how different 'terrain mode' selections alter behavior of various systems. The various systems are usually already in-place, they have to develop a default algorithm for making these things work together anyway, it's too easy to add alternate selectable algorithms and a selector button.
Then the marketing folks slap fancy names on it and use it to try to distinguish their vehicle from the similar competition in the cutthroat market - it's the big automotive game, ya know?
As to how effective or meaningful this stuff is for most (or any) drivers ..... well, that's another topic entirely