If you look at the grammar, user-defined-integer-literal can be octal-literal ud-suffix, and octal-literal is defined as either 0 or octal-literal ’opt octal-digit.
N4140 §2.14.8
user-defined-literal:
- user-defined-integer-literal
- […]
user-defined-integer-literal:
- octal-literal ud-suffix
- […]
N4140 §2.14.2
octal-literal:
0- octal-literal ’opt octal-digit
So 01'23s is a perfectly valid literal.