Help/dev: Add maintainer guide step for initial post-release development

Document steps for deprecating old policy settings and future-proofing
export files generated by `install(EXPORT)`.

Help/dev/maint.rst

@@ -299,3 +299,28 @@ announcing that post-release development is open::
   before staging or merging.
 
 .. _`CMake Discourse Forum Development Category`: https://discourse.cmake.org/c/development
+
+Initial Post-Release Development
+--------------------------------
+
+Deprecate policies more than 8 release series old by updating the
+policy range check in ``cmMakefile::SetPolicy``.
+Commit with a message such as::
+
+  Add deprecation warnings for policies CMP#### and below
+
+  The OLD behaviors of all policies are deprecated, but only by
+  documentation.  Add an explicit deprecation diagnostic for policies
+  introduced in CMake $OLDVER and below to encourage projects to port
+  away from setting policies to OLD.
+
+Update the ``cmake_policy`` version range generated by ``install(EXPORT)``
+in ``cmExportFileGenerator::GeneratePolicyHeaderCode`` to end at the
+previous release.  We use one release back since we now know all the
+policies added for that version.  Commit with a message such as::
+
+  export: Increase maximum policy version in exported files to $prev
+
+  The files generatd by `install(EXPORT)` and `export()` commands
+  are known to work with policies as of CMake $prev, so enable them
+  in sufficiently new CMake versions.