Well, Red Hat were quick to adopt CoreOS' appc, before a lot of companies ultimately united under the Open Container Project banner.

As for:

and it was (and, for now, still is) marked as an experimental utility that isn't for production purposes.

rkt used nspawn for a while, and it's still systemd-based.

The fact that Lennart was giving out presentations about nspawn specifically makes me believe it's very much intended to be used in production, as a "chroot on steroids".