Expand on backward compatibility

Author: zklaus

peps/pep-0780.rst

@@ -191,7 +191,7 @@ expressed as a frozen set of strings of the form ``f"{name}::{value}"``, where
 ``name`` is the name of the field and ``value`` is the string representation of
 its value.
 
-The ``sys_abi_info`` environment variable
+The ``sys_abi_info`` Environment Variable
 '''''''''''''''''''''''''''''''''''''''''
 To do this, we need to extend the grammar laid out in :pep:`508` and maintained
 in the :ref:`packaging:dependency-specifiers` and document the possible values,
@@ -303,6 +303,15 @@ However, the introduction of the feature has implications for a number of
 ecosystem tools, especially those which attempt to support examination of data
 in ``pyproject.toml`` and ``requirements.txt``.
 
+In particular, current tools generally work with a fixed list of environment
+variables, either in their own implementation (e.g. `uv <https://github.com/astral-sh/uv/blob/86e7b2e97a193ce3f506804be4c41c611191fa72/crates/uv-pep508/src/marker/tree.rs#L170>`_)
+or via an established package like `pypa/packaging <https://github.com/pypa/packaging/blob/8f13a4309aa620bf643cd508304f783b456e8c49/src/packaging/_tokenizer.py#L63>`_ (e.g. pip).
+Hence, it is principally impossible to add any new environment markers without
+making new packages using these markers incompatible with older versions of
+these tools.
+However, once these tools support the proposed marker variable, changes in its
+contents are handled gracefully.
+
 Audit and Update Tools
 ----------------------