It's one of those things that's hard to say 100%.

It's a myth that came around close to the films debut and was perpetuated by Gore Vidal (he is perhaps the earliest "documented source").

The term rosebud itself is (outside of Citizen Kane) used to refer to a clitoris, even back then. People did discuss sexual things, since language was a thing, so this isn't something taboo in that age. People always talked about their sex lives and their personal details.

Welles himself never said so in any interview(but was never asked), but did say he felt the film was unfair to Marion Davies and he regretted how it affected her.

Knowing this and how ROSEBUD is a focus in the film and knowing the history, how those outside of Welles and Hearst reacted to the film. It explains a lot.

It makes more sense than Welles just playing a newspaper tycoon: He clearly went after Hearst on a personal level- Which instigated Hearst's reaction to destroy the film's existence.

There is no evidence from anyone involved to explicitly say yes or no. The ones involved who say "No" all have agendas or weren't involved with the script; so their opinion is really just that... biased opinion.

I personally do believe it, but especially considering how much old Hollywood history is being rewritten.

20 years from reading this reddit post I wouldn't be surprised if people even forgot the origin of how the OSCAR got it's name. Its started as an inside joke (Betty Davis drunk thought the statue's ass looked like her ex-boyfriend's butt), but now the story is a secretary said "it looks like my uncle Oscar". That Secretaries name is never stated nor why anyone would care if this were true.

I personally always believe Alan Dwan's explanation for the OSCAR name same as this. It just explains so much and makes the most sense. Rewrite history all you want, I believe what makes the most logical sense.