Interesting that they took to the Google blog to announce this; is there a corresponding LKML post?
LKML would presumably be the place for Linux to announce when they adopt this.
The Google open source blog is among the several credible options for this post, since Google employs much of the core Git team, and this post discusses their experience deploying Git protocol v2 at Google.
As noted in the blog text, it's not in a released version of Git yet, just Git master branch. So maybe it'll appear on a dedicated Git announcement list, if any, once that happens.
It seems that https://groups.google.com/forum/#!forum/git-packagers is the closest thing to a formal announcement list that there is.
I presume Git 2.18 (the first release supporting protocol v2) will be announced via both channels once it's out.
Not yet, but presumably there will be a post like this: https://lkml.org/lkml/2018/4/2/425 when it is released. It is strange that the Google Blog is the first place to announce it through.