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I am very curious which program abused this and forced the creation of that rule.

Probably 2000/briddlebane [1]. But it is more like a guard against compatibility issues.

[1] https://www.ioccc.org/2000/briddlebane.c vs. https://www.ioccc.org/2000/briddlebane.orig.c

Well, technically the rule only talks about entries that "fail to compile". An entry that still compiles is fine, see rule 12. In practice this means the Unicode abuse like this is only allowed in strings.

When the rule was originally introduced in 2001 [1] it was a total ban. It seems that the rule was slightly relaxed in 2013 [2], but I think it still massively discourages any octet >= 128 because there is no portable way to set the input encoding (like GCC `-finput-charset`, which is ignored by Clang AFAIK).

[1] https://www.ioccc.org/2001/rules

[2] https://www.ioccc.org/2013/rules.txt

Aww. But also of course they've already addressed this.