The [cpp] section talks about "identifier"s frequently; often in a context where direct statements about grammar and macro expansion are made.

It would be appropriate to use \grammarterm{identifier} in many more places. For example:

A preprocessing directive of the form

# undef identifier new-line

causes the specified identifier no longer to be defined as a macro name. It is ignored if the specified identifier is not currently defined as a macro name.

- [cpp.scope] p2

This is one of the more extreme cases: statements directly about grammar rules are made, but "identifier" is not formatted as a grammatical term.