If it's free software it won't just disappear. If it's proprietary and hosted by an org that isn't making real money from it, that's a different story....
Is the development open though (serious question, I don't know how they do it with go)?
Look at android, it's a closed project that the occasionally release some source code for. If google decided to drop android then it's a critical blow because there isn't much of a community around it.
It depends what you mean by "open." Anyone can subscribe to the go-codereview mailing list. Anyone can submit a patch, but you need to a) use gerrit and b) sign a (digital) contribitors license agreement with Google.
It's "open", but it would almost certainly collapse entirely if Google decided to drop support for it. (There's no sign that they'll do that, but it's not a risk you take with a language like C.)
Even by the most lax standard you couldn't call the app store open; and that's kind of what's being shuttered here. The technology wasn't ever particularly interesting (not a bad thing), but the distribution model was. And that's entirely closed - they're not about to give anyone else permission to run even parts of the app store to anyone else, and that's that.