Implement large thread filter

config/config.exs

@@ -163,6 +163,8 @@
   allow_followersonly: false,
   allow_direct: false
 
+config :pleroma, :mrf_hellthreadmitigation, threshold: 10  
+
 config :pleroma, :mrf_simple,
   media_removal: [],
   media_nsfw: [],

lib/pleroma/web/activity_pub/mrf/hellthread_policy.ex

@@ -0,0 +1,15 @@
+defmodule Pleroma.Web.ActivityPub.MRF.HellthreadPolicy do
+  @behaviour Pleroma.Web.ActivityPub.MRF
+
+  @impl true
+  def filter(object) do
+
+    policy = Pleroma.Config.get(:mrf_hellthreadmitigation)
+
+    if (length(object["to"]) + length(object["cc"])) > Keyword.get(policy, :threshold) do
+      {:reject, nil}
+    else
+      {:ok, object}
+    end
+  end
+end