Is it possible to use the appengine/datastore package on the GAE flex environment?

There are two different datastore packages for Google App Engine:
google.golang.org/appengine/datastore and cloud.google.com/go/datastore.

The documentation for the appengine/datastore package uses the standard environment while the documentation for the cloud package uses the flex environment.

I have a Go application which uses the appengine/datastore package on the standard GAE environment. If I want to migrate this application from the standard environment to the flex environment do I need to switch to the cloud.google.com/go/datastore package, or can I continue using the appengine/datastore package? If not, why not?

You need to migrate to cloud.google.com/go/datastore.

From Migrating Services from the Standard Environment to the Flexible Environment:

> The App Engine flexible environment does not provide the API libraries
> present in the App Engine SDK. If you decide to migrate your
> application from the App Engine standard environment to the flexible
> environment, you must update your code to use services and APIs
> that are available across all Google Cloud Platform environments.

And from Cloud Datastore:

> You can access Google Cloud Datastore from anywhere using the
> Cloud Datastore API. Use the Google Cloud client libraries to
> store and retrieve data from Cloud Datastore.
>
> The same Cloud Datastore data is available regardless of if you use
> the App Engine libraries, the Google Cloud client libraries, or call
> the API directly.
>
> At this time ORM libraries that available in the standard environment
> such as ndb and Objectify are not supported outside of the
> standard environment.
>
> For more information, see the following guide:
>
> - Using Cloud Datastore