Apple keeps voice recordings of every Siri request and dictation you’ve made on your iPhone for 2 years. That is not good for privacy.
The staff here at NobodyisFlyingthePlane is evenly split between Android and iPhone so we well know that what one does so does the other. The slight differences in the length of time or methods of anonymization are irrelevant.
Everything you speak into your phone is stored on servers! Anonymizing the data is useless if the data itself contains personal info. A letter dictated might well include the name and address of the sender and recipient with tons of personal info squozen in between.