Add a note about deprecation of old versions

Author: SkowronskiAndrew

Analyzer/Util/SerializedFileDetector.cs

@@ -40,6 +40,11 @@ public class SerializedFileInfo
 ///
 /// Important: The header itself is always stored in big-endian format on disk,
 /// but the m_Endianness byte indicates the endianness of the actual data section.
+///
+/// DEPRECATION WARNING: The deprecation process for Version <18 (Unity 5.5 and earlier) has started in Unity 6.5.
+/// Initially this will be a warning, but upcoming versions of UnityDataTool and UnityFileSystem can be expected
+/// to lose the ability to open and read those files (apart from low level information exposed by the
+/// "serialized-file header" command).
 /// </summary>
 public static class SerializedFileDetector
 {