In Python 3 “second‑generation” App Engine, the legacy bundled services (Search, NDB, Memcache, etc.) only work if two things are true:
Your Flask/Django WSGI app is wrapped with the App Engine middleware, which plumbs the request’s API security ticket (X-AppEngine-API-Ticket) into the RPC layer.
app_engine_apis: true is set in app.yaml, which instructs the platform to attach that ticket to requests that need to call bundled services.
If either is missing, RPC calls made by google.appengine.* (like search.Index(...).get_range(...)) fail with “no active security ticket.”
Google’s docs explicitly say to install the App Engine services SDK, wrap your WSGI app, and enable app_engine_apis.
This isn’t a new deprecation of Search; rather, Search is a legacy API that’s still available to Python 3 apps via the services SDK.