IMO it's not. I've never given or taken a whiteboard interview where the I (or the candidate) was not allowed to either google things or to make assumptions about how certain things worked.
For example, if your solution involved generating all permutations of a sequence, and you didn't remember how to do that efficiently, I would let you make up a black box function that just does it. We might come back to this later in the interview, but I'm generally uninterested in whether you memorized an algorithm for it or remembered how to call the standard library function that does it.
That said, if you make up the black box function, I might dig into why you designed the function signature the way you did, and I'd do that to see whether you can talk about code design from a "other people will read and use and debug your code" perspective.
> if your solution involved generating all permutations of a sequence, and you didn't remember how to do that efficiently, I would let you make up a black box function that just does it
But sometimes, the interview question is generating all the permutations :(
For example, if your solution involved generating all permutations of a sequence, and you didn't remember how to do that efficiently, I would let you make up a black box function that just does it. We might come back to this later in the interview, but I'm generally uninterested in whether you memorized an algorithm for it or remembered how to call the standard library function that does it.
That said, if you make up the black box function, I might dig into why you designed the function signature the way you did, and I'd do that to see whether you can talk about code design from a "other people will read and use and debug your code" perspective.