@lunaryorn Just a quick update: Compatibility is now at 38% and there is a draft of JSCS migration guide. However, there are still around 30K downloads of JSCS every day.
I'm busy next 2 weeks, but after that we can check status again. Maybe we can wait until compatibility hits 60-80% or that migration guide is done. Doe to high number of downloads, the best solution IMHO is to extract JSCS checker into GH gist, then people can load that file into Emacs if they still need JSCS support. Maybe provide link to that gist in ChangeLog.