Gerrit has migrated to polymer 3 as of submitted of submitted of https://gerrit-review.googlesource.com/q/topic:%22bower+to+npm+packages+switch%22+(status:open%20OR%20status:merged).
The biggest difference between 2 and 3 is the changing of package management from bower to npm and also replaced the html imports with es6 imports so we no longer need templates in separate html files for polymer components.
As of now, we still support all syntax in Polymer 2 and most from Polymer 1 with the legacy layer. But we do plan to remove those in the future.
So we recommend all plugin owners to start migrating to Polymer 3 for your plugins. You can refer more about polymer 3 from the related resources section.
To get inspirations, check out our samples here.