But Python 2 on App Engine is not officially deprecated yet (e.g. not listed here: https://cloud.google.com/appengine/docs/deprecations https://cloud.google.com

But Python 2 on App Engine is not officially deprecated yet (e.g. not listed here: https://cloud.google.com/appengine/docs/deprecations https://cloud.google.com/appengine/docs/deprecations) and they have not sent the "[Action Required]" email (of which OP talks about) for that yet, so I'm not sure it counts. They are "just" recommending migration. I have a ~60 KLOC AppEngine Python 2 app (started in 2011), but I'll probably migrate that over only when the deprecation actually occurs, as: - that gives the python-ndb library more time to mature (performance and bugs), - easier migration paths for some of the non-py3 services I use (e.g. Images, Users, Memcache) might appear when deprecation actually occurs, either from Google or 3rd parties, - I'm in no hurry, - I will be surprised if Google gives less than 1 year of deprecation notice (Python 2.5 got 4 years from deprecation to termination). (If the APIs I use were available for py3, I would've probably already migrated.)