Rationale (July, 2009):

The characters in the template arguments are the characters comprising n, the integer literal, or f, the floating literal. As such, they are constrained by the grammar to be members of the basic character set, and no further normative restriction is needed.

5.13.9 [lex.ext] paragraphs 3-4 state in notes that the arguments to a literal operator template “can only contain characters from the basic source character set.” This restriction does not appear to occur anywhere in normative text, however.