It seems that the group behind Eclipse Scala IDE is not anymore developing the project and all commits to the GIT repository have ceased. The newest development is that the site scala-ide.org has stopped serving the plugin as the subdomain downloads.scala-ide.org is not online.
I honestly thought about trying to build the plugin form source but midway I gave up as the build errors were mounting one after another. Thankfully there is still hope. In the mailing list for the project there is a report regarding the problem and one of the team members has given a link for the update site zip file for the plugin. There is one slight issue though, as of this moment the linked .zip is not a valid update site for eclipse, one needs to extract its contents and provide to eclipse the path that is a level lower than the compressed root. In any case that's no big deal and after a few keystrokes one can have a new eclipse installation along with the Scala IDE plugin.
I value greatly the eclipse foundation and their work in providing a modern IDE for everyone free of charge. In contrast the market leader for developing java projects, IntelliJ IDEA, has a pretty steep price making it much less available to many businesses and individuals. Until there is a commercial competitor to eclipse we all need to offer our monetary support for the continuation of development for both the platform as well as the plugin components that are essential for our job.