This is overdue.
Various packages used by individual Grunt tasks used by local projects already throw warnings for the engine being unsupported.
These are not the packages for the legacy global tools in package.json in integration/jenkins.git:/tools aka slave-scripts, but the local package.json in projects like oojs, oojs-ui and VisualEditor.
As far as I can see these jobs seem to function fine, but it's really dangerous territory. The packages are explicitly indicated as requiring 0.10.x or higher (it's like executing a PHP application documented as requiring PHP 5.3 under PHP 5.1.x).
Both the upstream developer, their integration tests and the developers in our teams working locally already use Nodejs 0.10.x or higher, so this is waiting for something to fail.