This is basically all reasonable but entangled with the giant mess in T5791 and T9161.

Very briefly:

• Within mail handling, we receive relatively few requests for more powerful filtering/sorting tools, and relatively more requests to fix "too much mail" in some form.
• "Too much mail" is a complicated problem, discussion in T9161. Briefly, we have some evidence that the root cause of this problem is not that we send too much mail.
• Since I'm confident we can solve "insufficiently powerful tools" in a lot of ways but less confident we can solve "too much mail" at all, I want "too much mail" to be the primary problem informing how we approach mail delivery. Particularly, I don't want to finish T5791 (which adds a lot of powerful tools) in its current form if it might make "too much mail" worse, or make fixing it harder or more cumbersome.
• Many users use mail clients (primarily Gmail) which can not do any kind of header filtering.

If you want to fork, you can add a least the repository header trivially in DifferentialTransactionEditor->buildMailTemplate(), and it will likely remain stable until upstream support moves ahead. Something like this:

diff --git a/src/applications/differential/editor/DifferentialTransactionEditor.php b/src/applications/differential/editor/DifferentialTransactionEditor.php
index fabd251..8e60263 100644
--- a/src/applications/differential/editor/DifferentialTransactionEditor.php
+++ b/src/applications/differential/editor/DifferentialTransactionEditor.php
@@ -1188,9 +1188,16 @@ final class DifferentialTransactionEditor
     $subject = "D{$id}: {$title}";
     $thread_topic = "D{$id}: {$original_title}";
 
-    return id(new PhabricatorMetaMTAMail())
+    $mail = id(new PhabricatorMetaMTAMail())
       ->setSubject($subject)
       ->addHeader('Thread-Topic', $thread_topic);
+
+    $repository_phid = $object->getRepositoryPHID();
+    if ($repository_phid) {
+      $mail->addHeader('X-Custom-Repository-PHID', $repository_phid);;
+    }
+
+    return $mail;
   }
 
   protected function buildMailBody(

However, this won't work with Gmail and won't work with Herald outbound rules (T5791), and will almost certainly be obsoleted by a more powerful mechanism once T5791 moves further forward, so I don't want to bring it upstream because we'd get stuck with it even after it was obsolete.

(I think you should already be able to use the "Subject:" header to filter state changes?)

I'll leave this open for now and we can circle back after T5791 to make sure it covers everything, but I don't expect to make the changes you request outside of the scope of that, which is large on its own ("Build a more powerful Gmail in Phabricator so users who use Gmail can still do header filtering") and blocked on T9161 which is a very soft user experience task that we don't really even know how to move forward on.