DOCS-14667 retain historical versioning info

source/reference/versioning.txt

@@ -17,6 +17,9 @@ series:
 - Major Releases
 - Rapid Releases
 
+For MongoDB 4.4 and previous, MongoDB used a Production / Development
+versioning system. See :ref:`historical-releases`.
+
 Major Releases
 --------------
 
@@ -96,3 +99,24 @@ Database Tools
 Starting with MongoDB 4.4, the `MongoDB Database Tools
 <https://docs.mongodb.com/database-tools>`__ are released separately
 from the MongoDB Server, and use their own version numbering system.
+
+.. _historical-releases:
+
+Historical Releases
+-------------------
+
+For MongoDB 4.4 and previous, MongoDB versioning used a Production /
+Development versioning scheme, and had the form ``X.Y.Z`` where ``X.Y``
+refers to either a release series or development series and ``Z`` refers
+to the revision/patch number.
+
+- If ``Y`` is even, ``X.Y`` refers to a release series; for example,
+  ``4.2`` release series and ``4.4`` release series. Release series are
+  **stable** and suitable for production.
+
+- If ``Y`` is odd, ``X.Y`` refers to a development series; for example,
+  ``4.3`` development series and ``4.5`` development series.
+  Development series are **for testing only and not for production**.
+
+For example, in MongoDB version ``4.4.7``, ``4.4`` refers to the
+release series and ``.7`` refers to the revision.