How the heck can I turn off 'Summarize Previews' in Mail?

Ok, I did it.

There's an active off switch in System Settings>Siri>App Access>Mail.

This one seems to work...

You can turn Summarize Message Previews on or off in Apple Mail on your Mac through the app's settings.

How to Turn Message Preview Summaries On or Off

• Open the Mail app on your Mac.
• From the top menu bar, click Mail menu > Settings 
• Click the Viewing tab. 
• Select or deselect the box next to Summarize Message Previews.
  • When turned on: Apple Intelligence generates a short AI summary beneath unread emails and inside message threads.
  • When turned off: Mail displays the standard first few lines of the most recent message instead.

More info from Apple: Turn off and restrict access to Apple Intelligence features on Mac - Apple Support

How to Summarize an Individual Email Manually

• Select an email in your inbox and scroll up.
• Click the Summarize button at the top of the email to generate a quick breakdown of key points and action items

Note that if you are referring to the latter summarize 'button' as the waste of space, there is no separate setting to toggle off that feature. It is part of Apple Intelligence.

Apple Intelligence launched with macOS 15 Sequoia (and iOS 18), and since then it has become progressively more integrated with macOS (and iOS). On macOS 26 Tahoe, there was option to turn off Apple Intelligence.

On macOS Golden Gate, the toggle above has been removed, and Apple Intelligence is even more tightly integrated with macOS. Consequently, it is very difficult to turn off. Long-time forum member etresoft has written a helpful tip on how to Turn off Apple Intelligence in macOS 27. The title is somewhat misleading, Apple phrases it more accurately in their support article:

Turn off and restrict access to Apple Intelligence features on Mac

As Apple states, you can turn off and/or restrict access to some features of Apple Intelligence, but you cannot turn off Apple Intelligence altogether.

Apple Intelligence features can be restricted in a more comprehensive way by using a declarative configuration, but that requires device enrollment (i.e., management of the Mac with MDM) and that is beyond the typical processes with which most users are familiar. However, there is a script called Remove Mac AI that is available to execute those functions as a local Profile via a terminal command, if you are comfortable using it. At least one long-time member here has done so without issue. Use it at your own risk.

You can also share your opinion with Apple:

Feedback - macOS - Apple

Yes, I've done that. It doesn't work.

I then went to the 'Notifications' system setting, and can't turn them off there, either.