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Why not? It’s useful for people to be able to ping on a revision.

I was looking at this earlier, something simple like this may be enough but I don’t have a way to test it right now of course:

diff --git a/src/applications/differential/editor/DifferentialRevisionEditEngine.php b/src/applications/differential/editor/DifferentialRevisionEditEngine.php
index 74fee8221956..836e25fe51eb 100644
--- a/src/applications/differential/editor/DifferentialRevisionEditEngine.php
+++ b/src/applications/differential/editor/DifferentialRevisionEditEngine.php
@@ -128,6 +128,10 @@ final class DifferentialRevisionEditEngine
     }
 
     $is_create = $this->getIsCreate();
+    if ($is_create) {
+      throw new Exception('New revision creation is disabled, please use GitHub Pull-Requests.');
+    }
+
     $is_update = ($diff && !$is_create);
 
     $fields = array();

I would look into some data-driven approach: after we stop accepting new revision, we should be able to know how many revision are still updated every day/week and take it from there!