Even for C it's normal ways true: When size of structures have to be known to the user (if they are supposed to keep them on stack or mallox themselves or whatever) the "private" structure often ends up in a "private" header. (There are ways to still hide it, but they cause work to keep things proper)

And then there are cases where (often die to performance) you want inlining of some operations without relying on Link time optimisation, then implementation has to go to headers, too.