Interesting. In the 70's & 80's, 8-bit microcomputers had a the same problem as old teletypes: you couldn't type in some ASCII characters even when the computer could display them. (And many couldn't.) At least some home-grown tiny-C compilers allowed a Pascal-like "BEGIN" and "END" substitute, which I'm sure would be an abomination to Kernighan. Even Pascal itself could have a problem: the very popular TRS-80 Tiny Pascal used ( and ) instead of [ ] for arrays because neither brackets nor braces could be typed.

To summarize from the article for { and }:

Modern digraphs:

   <%  ,  %>. This is only one that looks symmetric

Less-modern trigraphs:

  ??<  ,  ??>

Unix v4 (in the teletype driver):

   \(  ,  \)

PDP-11 B:

   *(  ,  *)

PDP-7 B:

   $(  ,  $)

In other micro software (Advanced MuMath for the TRS-80), I have seen:

   <<  ,  >>    for [ , ]

   (<  ,  >)    for { , }

Back in the 80's, the joke among new learners of C and Unix is that the designers must have had a very bad keyboard where typing each character was painful, because every keyword or command was so short and cryptic. This article suggests a different reason: on their 36-bit Honeywell 6070, "four characters fit into a word", so there was incentive to fit in 1 machine word.

It also explains why they used the obscure characters {,},|, and ~ while never using the FAR more common # and @. In the Teletype driver, "#" is clear previous char, and "@" is clear current line. So unavailable for C. I will still curse the C designers to my dying day for picking * as the prefix operator for dereference pointer, when the more logical @ character was SITTING RIGHT THERE! On every keyboard! So now every newbie to C has to stop thinking "multiplication" when they see *.