After updating your iPhone to iOS 15.4 ( currently in beta[1] ), you'll be asked if you want to help improve Siri and dictation by allowing Apple to review recordings of voice interactions to improve both services. If you opt-out, your voice interactions with Siri or the voice dictation tool on your iPhone aren't recorded and shared with Apple.
However, Apple found a bug in iOS 15 that enabled the setting for some users who had previously opted out. In other words, recordings were being kept for some users who had opted out of the setting instead of being deleted. Apple has since deleted the erroneous recordings.
Once Apple discovered the bug, the company turned off the setting for "many" Siri users with the release of iOS 15.2. And with the second beta of iOS 15.4, Apple has fixed the bug, but in doing so, users will be asked if they want to opt-in or out after the update has been installed on their iPhone.
Apple provided ZDNet with the following comment: "With iOS 15.2, we turned off the Improve Siri & Dictation setting for many Siri users while we fixed a bug introduced with iOS 15. This bug inadvertently enabled the setting for a small portion of devices. Since identifying the bug, we stopped reviewing and are deleting audio received from all affected devices."
iOS 15.4 is currently available in beta. Apple published the second release of the upcoming update on Tuesday, which includes the fix. I just finished updating my to the latest beta and can confirm I was asked if I want to share my audio recordings with Apple as soon as I unlocked my iPhone after the update finished.