RedHat Linux has also been ported to this platform; it's probably ASCII, but not sure.
Oracle has a native client on zOS that does transparent conversion between ASCII and EBCDIC. It was probably built with such a compiler.
There was a paper that the original port of Research UNIX ran as a client on an IBM TSS/370 kernel, which likely was not ASCII.
https://gunkies.org/wiki/UNIX/370
RedHat Linux has also been ported to this platform; it's probably ASCII, but not sure.
Oracle has a native client on zOS that does transparent conversion between ASCII and EBCDIC. It was probably built with such a compiler.
There was a paper that the original port of Research UNIX ran as a client on an IBM TSS/370 kernel, which likely was not ASCII.
https://gunkies.org/wiki/UNIX/370