Most programmers manipulate characters as if they were ASCII, and that code will break if presented with EBCDIC. The C++ Standard is carefully worded to not assume ASCII encoding.

C++ presumably supports other encodings, but I've never heard of another one that will work. There's RADIX50, but that will never work with C++, for the simple reason that there aren't enough characters in it.

https://en.wikipedia.org/wiki/DEC_Radix-50

You can also see the EBCDIC support in oddities like the digraph support.