For software testing purposes, I use a Mac mini with four APFS boot volumes, one each for Big Sur, Monterey, Ventura, and Sonoma. These were all "fresh" macOS installs with no upgrades or data migration. For the most part, the Mac mini contains very little of my personal data. I do sign in with my Apple ID, but that's it. I never receive, read, or send email on that Mac. Nonetheless, while looking around in System Settings on Sonoma, I selected the Internet Accounts pane, clicked the Add Account button, and was shocked to see, in the list of Suggestions, five different email addresses of mine, only one of which was associated with my Apple ID. Two of the email addresses were Gmail (separate business and personal), one was a support address for my business domain, and one was an address with a third-party email provider. Four of the email address listed have never been used with any Apple service: not iCloud, not iMessage, not Facetime, not iTunes, not anything. I only ever use those email addresses in Apple's Mail app (on my MacBook Pro and my iPhone). So my question is, how in the world did Apple get all of those email addresses? Clearly the addresses are stored somewhere in Apple's iCloud, because they don't exist on disk on my Mac mini.
Curiously, I started using another third-party email provider a few months ago, but this email address was not in the list. I don't know whether that's because Apple doesn't have the email address or rather because the list of suggestions in the Internet Accounts Settings is limited to five items.
My use of iCloud is extremely limited. For many years I resisted iCloud entirely, but I finally caved in because many customers of my Safari extension StopTheMadness kept requesting iCloud support. In Settings, I allow StopTheMadness and my other app StopTheFonts to use iCloud and iCloud Drive (the latter of which is strangely a requirement for the CloudKit framework), but no other app is allowed, definitely not Mail or Contacts. And I don't use Siri at all. I've literally never used Siri, believe it or not, and disable everything Siri-related on all of my devices. I also disable analytics sharing with Apple.
Thus, I don't know how exactly Apple got a hold of my various email addresses. I'm both puzzled and troubled by this. Apple has advertised its commitment to privacy by claiming, "What happens on your iPhone, stays on your iPhone." Unfortunately, however, that doesn't appear to be true. Something that happened on my iPhone or my MacBook Pro somehow ended up on my Mac mini, and I want to know why. I want to know who or what is responsible for violating my privacy.
If you're looking to reproduce this issue yourself, you may need to do a fresh, clean operating system install on one of your devices. Otherwise, your list of Internet Accounts in Settings will already be populated, and of course you won't be offered suggestions for email accounts that are already in the list. That's probably why this privacy violation hasn't been noticed until now.