Google provides a list of App Engine features they've removed. [1] Beyond that there is also a somewhat undocumented phase of working-but-forgotten. Classic App Engine features like the datastore, memcache, Users API, Python 2, Go 1.11 etc go under this category. These are things that still work, but get no updates. Instead you get constant e-mails and other notifications about how you should redesign your app to work with the 2nd generation App Engine system. Which means Firestore (in datastore mode) instead of datastore. Memorystore instead of memcache. Your own solution instead of the Users API etc.
Yes, this. Thank you for explaining it better than I did. I loved this with a real production python 2 app, and my take-away, be very careful building against proprietary systems like classic app engine and all of its services.
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[1] https://cloud.google.com/appengine/docs/deprecations