I have a formal proof for you that it is a non-concept. If code can be read and interpreted by a computer, it means it can in principle be read by a human. There are of course some edge cases like obfuscated JavaScript or binary executable that some people are able to read and understand.
The question comes down to being reasonably readable and we are back to square one: "reasonable" is very relative. In my early days I could read 8086 binary code (in hex) and understand what it does, it was literally at the very edge of readability but it wasn't unreadable.
You are using a different definition of readable than most people are. Most people are using it to mean "the target audience can read and understand the code, and do so in a way/context that allows them work with it". Your definition seems to be "can read the symbols on the screen".
I can read Assembly. I can, in some cases, figure out what that Assembly is doing. I can not, however, work productively with it. I can read Assembly but would not consider it readable.
The question comes down to being reasonably readable and we are back to square one: "reasonable" is very relative. In my early days I could read 8086 binary code (in hex) and understand what it does, it was literally at the very edge of readability but it wasn't unreadable.