Seriously, at this point C should be considered dangerous and should be actively discouraged. I would even go so far as to legislate it. We can't have this sort of thing in 2019.
We don't really have this in 2019. All modern C compilers disable trigraphs by default, so you have to go out of your way to see this in person.
They only existed because really old terminals sometimes didn't have various punctuation characters used by C so they needed the ugly workaround so people trapped on those terminals wouldn't be left out in the cold.
The sad thing is that in safety critical embedded systems often the only choice you have is C and C++ because nobody would shell out for ADA developers. Rust would be a contender as well, however I'm not sure how many embedded targets that aren't major (like arm) even have a target.
They only existed because really old terminals sometimes didn't have various punctuation characters used by C so they needed the ugly workaround so people trapped on those terminals wouldn't be left out in the cold.