The next major version of google-cloud-cpp (the 4.x series) is expected to
include the following breaking changes.
Pubsub: remove legacy admin APIs
We will remove the hand-written versions of the Pub/Sub admin APIs. These have
been superseded by versions generated automatically from the service
definitions. The new APIs can be found in the
google/cloud/pubsub/admin
tree and within the google::cloud::pubsub_admin namespace. Starting with the
v2.23.0 release, and depending on your compiler settings, using the old
classes/functions may elicit a deprecation warning. See #12987 for more
details.