For years, Overviewer has been using the Google Maps API to display its rendered maps. This means we just use their JavaScript files, and none of their data. In fact, we wouldn't even be allowed to ship their JavaScript file ourselves; the JS file is requested from Google on a page load.
However, Google started restricting their API, forcing all new Overviewer users to get a (free) API key from Google just to use one JS file. Just a few days ago, Google has announced that they will require everyone to sign up with a credit card to make use of the Google Maps API, which is unacceptable for our use-case.
Therefore, Overviewer now uses Leaflet, a free as in freedom map framework. In addition to our users no longer requiring Google Maps API keys, this also means that maps can now be viewed without an internet connection, and are smoother to use. Furthermore, we're now no longer at the whims of Google, who have proven to be bad to rely on.
Some features may still be missing from the Leaflet version, but all in all we believe it is a very usable experience. If you notice deficiencies, feel free to submit a pull request or open an issue.