Languages like C that have a single official for loop syntax are a bit easier to remember, in part because it's a syntax I have previously used on a whiteboard rather than exclusively in actual code.
So if a candidate can't write a loop on a whiteboard, maybe let them try in a non-IDE text editor, or have them glance at a formal grammar for the language as a refresher that won't give away the game to someone who can't actually code.
And if I start showing them an EBNF grammar for the language, now that's TWO languages that you will show you don't remember rather than one. I find this even more absurd; you don't remember anything of the syntax of the language you used at work for years, but you think you can rediscover it from the FORMAL language specification in a whiteboard?
The problem is when you apparently _forget_ the syntax for a function declaration in a programming language you claim N years of experience in your CURRENT position. This is just absurd. Our you suddenly get completely stuck with counting odd numbers on a sequence or the like. And this happens surprisingly frequently. I just cannot believe you can have this type of anxiety (I am extremely shy myself) outside your first or maybe second interview.