Yeah, this one caught me by surprise also. It is documented, although not in a very visible way: https://cloud.google.com/appengine/docs/flexible/python/upgrading#domain_update:
Traffic is now served from the appspot-preview.com domain instead of the > appspot.com domain. All traffic from the App Engine flexible environment > will be automatically redirected to the new domain. All App Engine Standard > traffic will continue to be through appspot.com. When the App Engine > flexible environment is generally available (GA), users will be able to use > the appspot.com or appspot-preview.com domains for routing traffic. -- You received this message because you are subscribed to the Google Groups "Google App Engine" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/google-appengine. To view this discussion on the web visit https://groups.google.com/d/msgid/google-appengine/b2027383-49e1-4ef7-b870-b70fa07c9199%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
