Apple's systems are under constant attack. I don't know precisely if Siri itself has been a subject of attack or not, but I think a good default position would be to assume it has. However, because the Siri data is not associated with the overall Apple account, while we consider it very sensitive and will strive to protect it, it would also be challenging to gather an individual user's data out of the Siri system, even if it were breached.
On May 29th, 2019. See this statement in context.