Was this done proactively or reactively after many months of negative feedback? If it was the former that gives me a lot more faith in app engine after trialing in some years ago and giving up on it.
Proactively. It was in the initial email about the Go 1.9 turndown sent to users. We definitely understand that any code change is painful, no matter how trivial. Supporting legacy APIs on a modern Go runtime seemed like the right balance between keeping up with the language community while bringing along our existing users.