As of March 2018, the Slic3r repositories hosting the main codebase and the manual were moved to the https://github.com/slic3r organization account. GitHub provides redirects for all the existing URLs so nothing broke because of this.
Since the birth of the project, the code was hosted in my personal account. However Slic3r has grown far beyond a personal project this change was necessary for giving it the necessary independence that a mature open source project requires.
Many thanks go to Nick Semansky from San Francisco, who previously owned the github.com/slic3r account and kindly donated it to the project making this important step possible.