Deleted Answer

> Does this apply to user literal operators?

Yes and No.  If we look at the grammar at the start of [\[over.literal\]][1] we have

	operator string-literal identifier
	operator user-defined-string-literal
	
as the different kinds of literals.  The `operator string-literal identifier` way is when you have a literal like

	operator "" _some_name
	
where there is a space bewteen the `""` and the name and in this case `_some_name` is an identifier and per [\[lex.name\]/3][2] so it cannot be used.

Then there is

	operator user-defined-string-literal
	
and that is defined in [\[lex.ext\]][3] to be

	string-literal ud-suffix

that gives you an operator like

	operator ""_some_name
	
where there is no space between the `""` and the name of the literal.  If we then look at [\[over.literal\]/1][4] we have

> [...]The `ud-suffix` of the `user-defined-string-literal` or the `identifier` in a `literal-operator-id` is called a *literal suffix identifier*. **Some literal suffix identifiers are reserved for future standardization; see [usrlit.suffix]**. A declaration whose literal-operator-id uses such a literal suffix identifier is ill-formed, no diagnostic required.

<sup>emphasis mine</sup>

and that says that only the names defined in [usrlit.suffix] are reserved.  If we look at [\[usrlit.suffix\]][5] we find

> Literal suffix identifiers that do not start with an underscore are reserved for future standardization.

and since `operator ""_some_name` starts with an underscore, it is allowed.

  [1]: https://timsong-cpp.github.io/cppwp/over.literal
  [2]: https://timsong-cpp.github.io/cppwp/lex.name#3
  [3]: https://timsong-cpp.github.io/cppwp/lex.ext
  [4]: https://timsong-cpp.github.io/cppwp/over.literal#1
  [5]: https://timsong-cpp.github.io/cppwp/usrlit.suffix