I think it might apply only to WATCHLISTS.

I can't reproduce this. See here:
https://chromium-review.googlesource.com/c/527666/

That repo has a watchlists file which automatically adds agable+experimental@ to changes to the agable/ subdirectory. When I uploaded the change, that address was automatically added, but no email was sent. I then sent a message from the UI manually, and it was sent as expected. I then uploaded a second patchset, and it was properly uploaded with notify=NONE, and no new patchset email was sent. Attached is the screenshot of the email thread; it only has one email in it.

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Marking WontFix because
a) I can't reproduce this case
b) Even if this is happening, I think this will be fixed by  issue 718513 

This is still happening and I get a lot of email from chris that he probably doesn't mean to send (Chris Harrelson uploaded patch set #2 to this change. Chris Harrelson uploaded patch set #3 to this change. Chris Harrelson uploaded patch set #4 to this change.). I bet Chris gets similar emails from me.

Are emails supposed to go out before a patchset is uploaded for review? Is this just the behavior of watchlists?

I don't understand what's happening here, and I can't reproduce it. For example, here is a change which added chromium-reviews to CC due to a WATCHLIST:
https://groups.google.com/a/chromium.org/forum/#!msg/chromium-reviews/-uDGtnH6nTM/3pMNdaRaBwAJ;context-place=forum/chromium-reviews

The only "uploaded patchset" message in that thread is for patchset #2, which was a commit message edit in the browser, for which we can't control the email message (and the bug to silence that is issue gerrit:6543).

So I have a two theories for what's going on here:
1) Chris Harrelson always sets "-s" or "--send-mail" when running git-cl-upload, thus overriding our automatic email suppression
2) A lot of the emails you're getting are actually for commit message edits, not new patchsets uploaded via git-cl

Can you please confirm that neither of those is the case before I dig in further?

What about emails like this:
https://groups.google.com/a/chromium.org/d/msg/chromium-reviews/_hyA6YAxJr0/n28IpT_GBgAJ

As you can see from https://chromium-review.googlesource.com/c/559859/2..3, Patchset 3 (which generated that email) was an edit of the commit message made in the browser. That's case (2) in my comment above.

Okay I think the bulk of these are commit message edits then. I have starred https://bugs.chromium.org/p/gerrit/issues/detail?id=6543.

I thought a CL in WIP mode would not trigger any emails. Is that possible?

https://bugs.chromium.org/p/gerrit/issues/detail?id=6615 will make commit message updates not send any emails in WIP mode.

https://bugs.chromium.org/p/gerrit/issues/detail?id=6543 is considering making them not send any emails at all ever, although I doubt that one will get resolved (commit message updates can be important, especially when doing things like adding TBR= lines).