Merge pull request #7173 from kenjis/fix-docs-BC-note-depreacted-Config

docs: add about removal of deprecated Config items

user_guide_src/source/installation/backward_compatibility_notes.rst

@@ -13,5 +13,7 @@ However, the code is not mature and bug fixes may break compatibility in minor r
 What are not Breaking Changes
 *****************************
 
+- The deprecated Config items are not covered by backwards compatibility (BC) promise. It may be removed in the next
+  **minor** version or later.
 - System messages defined in **system/Language/en/** are strictly for internal framework use and are not covered by backwards compatibility (BC) promise. If developers are relying on language string output they should be checking it against the function call (``lang('...')``), not the content.
 - `Named arguments <https://www.php.net/manual/en/functions.arguments.php#functions.named-arguments>`_ are not covered by backwards compatibility (BC) promise. We may choose to rename method/function parameter names when necessary in order to improve the  codebase.