This is the kind of comment that reduces me to complete and utter baffled disbelief: that the extraordinary cinematic experience of a work like Citizen Kane can be reduced in some minds to a single, rather hackneyed mcguffin, which if known in advance, in some way "spoils" the film.
We do not write a good encyclopedia by pandering to that kind of illiteracy (a word that used in this context is, I think, doubly appropriate).
The film was written in such a way as to keep the fact secret until a particular point. If you already know the secret then you are seeing the film in a way other than that which the writer intended. If the film wasn't spoilt by knowing the secret early, why would the writer have bothered to keep it secret?