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.
Since most of Gerrit plugins are treated as sub modules and part of the Gerrit workspace when develop, dependencies of plugins are also defined and installed from Gerrit WORKSPACE, currently most of them are bower_archives. When moving to npm, if your plugin requires dependencies, you can have them added to your plugin's package.json and then link that file to plugins/package.json in gerrit. Then use @plugins_npm//:node_modules to make sure rollup_bundle knows the right place to look for. More examples from image-diff plugin, change 271672.