The App Engine Task Queue REST API (v1), was turned down as of February 20, 2018. The separate product Cloud Tasks provides a REST API that you can use to add tasks from a second generation App Engine standard environment runtime, any of the App Engine flexible environment runtimes, or even from entirely outside of App Engine.
For first generation runtimes, the google.golang.org/appengine/taskqueue package from the App Engine API is still available for use in your application.