Document the support policy for optional packages (#3616)

Co-authored-by: Wei Ji <[email protected]>

README.md

@@ -191,14 +191,7 @@ Other official wrappers for GMT:
 ## Minimum supported versions
 
 PyGMT has adopted [SPEC 0](https://scientific-python.org/specs/spec-0000/) alongside the
-rest of the scientific Python ecosystem, and therefore:
-
-- Support for Python versions be dropped 3 years after their initial release.
-- Support for core package dependencies (NumPy, pandas, Xarray) be dropped 2 years after
-  their initial release.
-
-Similarly, the PyGMT team has decided to discontinue support for GMT versions 3 years
-after their initial release.
-
-Please see [Minimum Supported Versions](https://www.pygmt.org/dev/minversions.html) for
-the minimum supported versions of GMT, Python and core package dependencies.
+rest of the scientific Python ecosystem, and made a few extensions based on the needs of
+the project. Please see [Minimum Supported Versions](https://www.pygmt.org/dev/minversions.html)
+for the detailed policy and the minimum supported versions of GMT, Python and core
+package dependencies.

doc/minversions.md

@@ -18,14 +18,25 @@ myst:
 # Minimum Supported Versions
 
 PyGMT has adopted [SPEC 0](https://scientific-python.org/specs/spec-0000/) alongside the
-rest of the scientific Python ecosystem, and therefore:
+rest of the scientific Python ecosystem, and will therefore:
 
-- Support for Python versions be dropped 3 years after their initial release.
-- Support for core package dependencies (NumPy, pandas, Xarray) be dropped 2 years after
-  their initial release.
+- Drop support for Python versions 3 years after their initial release.
+- Drop support for core package dependencies (NumPy, pandas, Xarray) 2 years after their
+  initial release.
 
-Similarly, the PyGMT team has decided to discontinue support for GMT versions 3 years
-after their initial release.
+In addition to the above, the PyGMT team has also decided to:
+
+- Drop support for GMT versions 3 years after their initial release, while ensuring at
+  least two latest minor versions remain supported.
+- Maintain support for [optional dependencies](/ecosystem.md#pygmt-dependencies) for at
+  least 1 year after their initial release. Users are encouraged to use the most
+  up-to-date optional dependencies where possible.
+
+:::{note}
+The SPEC 0 policy is enforced on a best-effort basis, and the PyGMT team may decide to
+drop support for core (and optional) package dependencies earlier than recommended for
+compatibility reasons.
+:::
 
 | PyGMT Version | GMT | Python | NumPy | pandas | Xarray |
 |---|---|---|---|---|---|