Whenever I install a new version of macOS, I always disable Siri in the setup screen. In fact I always disable Siri on my iOS devices too. I have no interest in Siri, and I don't want my devices to phone home to Apple with so-called "anonymous" data that always turns out to be less anonymous than claimed. I expected that the OS would respect my setup screen choice, and indeed if I look at the Siri pane in System Preferences, everything looks disabled.
Looks can be deceiving though. The problem is that Apple has a much broader conception of "Siri" than most Apple users. We tend of think of Siri as the computer voice that responds to "Hey Siri" requests, sometimes usefully and sometimes ludicrously. But I didn't expect Siri to make requests of me, to say "Hey Jeff". Nonetheless, I have occasionally seen Siri Suggestions, specifically in Mail app. The other day I got an email recounting something that happened "on Thursday", and "Siri" suggested that it found an event in the email, even though there was no such event, and moreover, I don't even use any Apple software to handle events. (I long since ditched iCal/Calendar for BusyCal.) So I asked around (not from Siri) if there was a way to disable this in Mail — perhaps a hidden
defaults command? — and a helpful person (not Siri) pointed me towards the Siri Suggestions & Privacy button that you can see in the above screenshot, but which I never bothered to investigate, because why would I, when Siri is disabled?
Although this (postmortem) screenshot shows the checkboxes disabled, everything was enabled for every app when I first opened the… I guess it's a sheet? It seems to me like a pretty obscure place to put these preferences, but at least now I knew how to turn them off. Problem solved! The story would have ended there if I hadn't decided to press About Siri & Privacy while I was already exploring previously unknown territory.
There's a bunch of stuff about Ask Siri and Dictation. At the very end you see Siri Suggestions, but you don't want to click "Learn More…" because that's the wrong thing. In typical Apple fashion nowadays, this spectacularly bad UI has a disclosure widget that you want to click to learn about Siri Suggestions.
I'm going to reproduce the full text of that section below.
Siri is designed to protect your information and enable you to choose what you share.
Siri Analyzes How You Use Your Devices and Apps to Provide Personalized Suggestions and Better Search Results Using Local, On-Device Processing, and Syncs Across Your Devices with End-to-End Encryption Using iCloud
Siri uses local, on-device processing to learn how you use your devices and apps in order to personalize your experience. Using information stored on your device, such as your Safari browsing history, emails, messages, notifications, and contacts, as well as information contributed by other installed apps, Siri can provide suggestions in Spotlight, Look Up, Safari, apps, and more.
Siri uses iCloud to sync personalization across your Apple devices using end-to-end encryption.
To Make Suggestions and Search Results More Relevant, Some Information Is Sent to Apple and Not Associated with You
When you use Siri Suggestions or Look Up, or type in Search, Spotlight, Safari search, or #images search in Messages, any information sent to Apple does not identify you, and is associated with a 15-minute random, rotating device-generated identifier. Your device may send information such as location, topics of interest (for example, cooking or basketball), your search queries, suggestions you have selected, apps you use, and related device usage data to Apple. This information does not include search results that show files or content on your device. If you subscribe to music or video subscription services, the names of these services and the type of subscription may be sent to Apple. Your account name, number, and password will not be sent to Apple.
This information is used to process your request and provide more relevant suggestions and search results, and is not linked to your Apple ID, email address, or other data Apple may have from your use of other Apple services.
Aggregated information may be used to improve other Apple products and services. Common search queries may be shared with a web search engine to improve search results.
Search Engine Suggestions in Safari
Safari has a single field for searches and web addresses so you can browse the web from one convenient place. When Search Engine Suggestions are enabled, Safari will ask your selected search engine for suggestions based on what you’ve typed.
Preload Top Hit in Safari
With Preload Top Hit enabled, as soon as Safari determines a Top Hit based on your bookmarks and browsing history, Safari will begin loading the webpage in the background. If you disable this option, the page will load normally.
You Have Choice and Control
If you do not want Siri Suggestions to send your information to Apple, you can disable Siri Suggestions. Go to System Preferences > Spotlight > Search Results and deselect “Spotlight Suggestions” and “Allow Spotlight Suggestions in Look Up.” You can disable Safari Suggestions in Safari by going to Safari > Preferences > Search, and deselecting Include Safari Suggestions.
You can stop apps from contributing information to personalize Siri by going to System Preferences > Siri, then clicking Siri Suggestions & Privacy. Select an app from the list and deselect Learn from this App.
You can disable Location Services for Siri Suggestions by going to System Preferences > Security & Privacy > Privacy, then selecting Location Services. Click the lock to make changes and enter your password, click Details next to System Services, and deselect Location-Based Suggestions. If you turn off Location Services on your device, your location will not be sent to Apple. To deliver relevant search suggestions, Apple may use the IP address of your internet connection to approximate your location by matching it to a geographic region.
If you do not want Siri personalization to sync across your devices, you can disable Siri by going to System Preferences > Apple ID > iCloud and deselecting Siri in “Apps on this Mac using iCloud.”
I believe that I understand what this is saying, but the most confusing and maddening thing about it is that not only are the Siri Suggestions preferences themselves somewhat hidden, but the procedures — multiple procedures! — required to stop Siri Suggestions from phoning home to Apple are hidden in entirely different places. It's like an obstacle course of apps, preference panes, and buttons. Why isn't this all in one obvious place? And that's just Siri Suggestions, not Ask Siri itself, which also has other privacy preferences. There's one I can't find mentioned even in that lengthy document, so I'll end with another screenshot FYI.
Nothing in this blog post is "new" or "secret", but it's all so obscure and convoluted that it might as well have been secret. If you wanted to hide something in plain sight, as it were, with plausible deniability of privacy violation, this is how you'd do it. It's not how you'd do it if you wanted to be clear and upfront. "You Have Choice and Control", if you can discover it LOL.