Prevent Gmail Threaded Conversation on Notification emails

You may be in luck now, check out this article - https://gsuiteupdates.googleblog.com/2019/03/threading-changes-in-gmail-conversation-view.html - because of the new requirement to have the mail header reference a previous email message ID, your system generated emails are probably no longer threading if your situation is like mine. Good for you, but that wasn't what I wanted for my use case! Enjoy.

Google seems to weigh the subject line pretty heavily in their threading heuristic, so there doesn't appear to be much that you, as a sender, can do about it without unique-ifing the subject lines somehow.

Adding a timestamp to the subject line seems to defeat the threading -- do you think you could get your users to buy into that?

On the recipient's side, they could use the IMAP interface to bypass gmail's threading. And I hear Google is open to giving users the option to disable the "conversations" feature -- it's obvious that there are a lot of people out there who hate it!