I believe we are up to plasma 5.5.3 at this point and still no backport to 15.10. This is highly unusual for Kubuntu which may not be on the bleeding edge but is usually on the leading edge. 5.5 has been out for over a month and I believe has some fixes in it that I am waiting for.
Is there some problem with 5.5 that we are being protected from? Are the changes that significant?