lowest version that I've found in my files is 65560

Author: Alica228

WowPacketParserModule.V10_0_0_46181/Parsers/InstanceHandler.cs

@@ -152,7 +152,7 @@ public static void HandleMultiFloorNewFloor(Packet packet)
         public static void HandleEncounterstart(Packet packet)
         {
             packet.ReadInt32("DungeonEncounterID");
-            if (ClientVersion.AddedInVersion(ClientVersionBuild.V12_0_1_65818))
+            if (ClientVersion.AddedInVersion(ClientVersionBuild.V12_0_0_65560))
                 packet.ReadInt16<DifficultyId>("DifficultyID");
             else
                 packet.ReadInt32<DifficultyId>("DifficultyID");

WowPacketParserModule.V6_0_2_19033/Parsers/InstanceHandler.cs

@@ -208,7 +208,7 @@ public static void HandleEncounterStart(Packet packet)
         public static void HandleEncounterStop(Packet packet)
         {
             packet.ReadInt32("EncounterID");
-            if (ClientVersion.AddedInVersion(ClientVersionBuild.V12_0_1_65818))
+            if (ClientVersion.AddedInVersion(ClientVersionBuild.V12_0_0_65560))
                 packet.ReadInt16<DifficultyId>("DifficultyID");
             else
                 packet.ReadInt32<DifficultyId>("DifficultyID");