From the announcement:
Big changes have come to the Geolocation plugin for Omeka Classic with today’s release of version 3.0!
This new version uses the service Leaflet to provide maps, rather than Google Maps. Not only does this mean you no longer need to worry about registering for an API key, but you can now choose from a wide variety of base maps, including some without labels or human-defined borders. In addition, technically skilled users can use the service MapBox to create custom map layers for use in their Omeka Classic site.
Read the full announcement here.