To follow-up, the build of stable extensions should now be restored.
Following pull request Slicer/DashboardScripts#70, the Slicer stable build scripts have been updated.
Earlier this afternoon, the Slicer Stable release script was triggered with the environment variable run_ctest_with_upload set to FALSE. Once completed, this will ensure the build tree required to build the Stable Slicer extensions is available.