Any mapping software built using that kit will stop working on December 12th, 2015. As the company explains, it's simply a matter of security. Chrome and Firefox are phasing out support for the old, vulnerable Netscape plugin format that Earth uses for third-party web apps, and Google doesn't want to leave people open to attack.
This isn't necessarily the end for broad Earth support, mind you. Google notes that "3D is in [its] blood," and is teasing that there's more in the pipeline. It's not clear that this includes a new toolbox, but you may not have to worry about a dearth of clever maps just yet.
