What this means concretely is that nplan should now be installed by default on all images, part of ubuntu-minimal, and dropped ifupdown at the same time.
For the time being, ifupdown is still installed by default due the way debootstrap generates the very minimal images used as a base for other images -- how it generates its base set of packages, since that depends only on the Priority: field of packages. Thus, nplan was added, but ifupdown still needs to be changed (which I will do shortly) to disappear from all images.
The intent is that nplan would now be the standard way of configuring networks. I've also sent an email about this to ubuntu-devel-announce@.
I've already written a bit about what netplan is and does, and I have still more to write on the subject (discussing syntax and how to do common things). We especially like how using a purely declarative syntax makes things easier for everyone (and if you can't do what you want that way, then it's a bug you should report).
MaaS, cloud-init and others have already started to support writing netplan configuration.
The full specification (summary wiki page and a blueprint reachable from it) for the migration process is available here.
While I get to writing something comprehensive about how to use the netplan YAML to configure networks, if you want to know more there's always the manpage, which is the easiest to use documentation. It should always be up to date with the current version of netplan available on your release (since we backported the last version to Xenial, Yakkety, and Zesty), and accessible via:
man 5 netplan
To make things "easy" however, you can also check out the netplan documentation directly from the source tree here:
There's also a wiki page I started to get ready that links to the most useful things, such as an overview of the design of netplan, some discussion on the renderers we support and some of the commands that can be used.
We even have an IRC channel on Freenode: #netplan
I think you'll find that using netplan makes configuring networks easy and even enjoyable; but if you run into an issue, be sure to file a bug on Launchpad here: