Impact launched almost 8 years ago. The last update was published in 2014. While Impact still works nicely in modern browsers, it lacks support for better graphic and sound APIs that are now available. I felt increasingly bad for selling a product that is hardly maintained or improved.
So as of today Impact will be available completely for free, published under the permissive MIT License.
Impact's source is available on github.com/phoboslab/impact
Thanks anyone who bought a license in the last few years!