5.11 [lex.name] paragraph 3 says,

In addition, some identifiers are reserved for use by C++ implementations and standard libraries (_N4140_.17.6.4.3.2 [global.names]) and shall not be used otherwise; no diagnostic is required.

There is no corresponding mention in 5.13.9 [lex.ext] of the restrictions on user-defined literal suffixes in 16.4.5.3.6 [usrlit.suffix]. Furthermore, considering the likelihood of adding hexadecimal floating-point literals, whose syntax overlaps that of user-defined literals except for that restriction, it would be a good idea to require a diagnostic for a violation of that rule.