Add a deprecation notice for the legacy Python client

We can point at this in announcements and if users ask questions that it
covers.

docs/posts/deprecate-py-legacy.md

@@ -0,0 +1,75 @@
+# :warning:  Deprecating `lakefs-client`, the legacy Python :snake: Client SDKs
+
+`lakefs-client` is now deprecated, and we will shortly publish its last
+release to PyPI.
+
+## What is changing?
+
+Ever since we released lakeFS 1.0, we have supported these two SDKs to program for lakeFS with Python:
+- [`lakefs`, the lakeFS High-Level Python SDK][pypi-lakefs-hi-lvl].  This is offers a structured easy way to use lakeFS from lakeFS.
+- [`lakefs-sdk`, the lakeFS HTTP API][pypi-lakefs-sdk].  This is
+  auto-generated from the lakeFS OpenAPI spec.  It is guaranteed to support
+  all lakeFS API actions.  And it is covered by the lakeFS SDK interface
+  guarantees.
+
+**These clients will continue.**
+
+Before lakeFS 1.0 we would publish only:
+
+- [`lakefs-client`, a legacy older version of the lakeFS HTTP
+  API][pypi-lakefs-legacy].  This too was auto-generated.  However it cannot
+  provide interface guarantees.  **At no time could this client ever provide
+  forward- or backward- compatibility at the source code level.**  
+
+We have continued to publish `lakefs-client` since the 1.0 release, in order
+to provide continued support for existing users of this legacy Python SDK
+client.  But there are better alternatives, and its continued presence only
+confuses our users.
+
+Now it is time to deprecate this client, and we will stop publishing it at
+or after the version 1.40 release of lakeFS.
+
+## FAQ
+
+### What will happen to existing users of `lakefs-client`?
+
+All existing uses will continue to work.  Obviously we will not be pulling
+these published versions because of this change.  We will stop publishing
+new versions of these clients.
+
+### how do I migrate from using `lakefs-client` to `lakefs-sdk`?
+
+Migration is covered by [Migrating to 1.0][lakefs-py-migration].  In most
+cases the changes are quite simple.
+
+### Will new versions of `lakefs-client` be published?
+
+No.
+
+Version 1.40 will be the last release of lakeFS to include a new version of
+the legacy client.  In order to use any _new_ features of the lakeFS API,
+you will need to upgrade your programs to use `lakefs-sdk`.
+
+### Will programs using old versions of `lakefs-client` continue to work with new versions of lakeFS?
+
+Yes.
+
+A program that uses any 1.x version of `lakefs-client` still uses 1.x
+versions of lakeFS APIs.  Its usage of those APIs is still covered by the
+lakeFS interface guarantees, so it will continue to work with future 1.x
+releases.
+
+### What about the legacy Java SDK generated client?
+
+That is also going away.  However we see much less usage of it.
+
+### Where can I ask another question not covered here?
+
+As always, our Slack htttps://lakefs.io/slack is the best place to interact
+with the lakeFS community!  Try asking on our `#dev` channel
+
+
+[pypi-lakefs-hi-lvl]:  https://pypi.org/project/lakefs/
+[pypi-lakefs-sdk]:  https://pypi.org/project/lakefs-sdk/
+[pypi-lakefs-legacy]:  https://pypi.org/project/lakefs-client/
+[lakefs-py-migration]:  https://docs.lakefs.io/project/code-migrate-1.0-sdk.html#migrating-sdk-code-for-python