Exactly what the subject says: plasma-desktop has migrated to testing, it will be there as soon as the mirrors are synced.
It is highly recommended for those in testing to upgrade today, although you might need to wait your mirror.
Also note that the gcc5 transition has started, that means we are not going to upload new stuff in some days (possibly a week, maybe even more) to facilitate this **huge** transition.
We really hope you enjoy the packages. If you filed a bug and after updating you findd that it is not there anymore then please follow-up the bug and state this. This is really **highly** appreciated.
Once again, sorry for the inconveniences!