There is an upside: complex header files often contain a bunch of #ifdef … #endif blocks, especially when dealing with cross-platform support. Since indentation is generally not used there, they can be rather hard to read, especially when nested.

Not having the include guard use the same mechanism makes these a bit easier to read and removes a little bit of cognitive load. (And every bit of cognitive load counts since that's a very hard limit of people's brains.)

Personally (and for the projects I work on), I couldn't care less whether it exists in ISO C or not. I have a list of compilers and platforms I want/need to support. But of course your project may be different.