I have never heard of "German Measles" being offensive. That was the common term used during my childhood, circa 1950s, to distinguish it from measles. (Those were called the "big red measles".) I recall that about 1969, when the vaccine against it became available, I started hearing the term "rubella" instead. "Rubella" became an even more common usage when the triple vaccine, MMR (Measles, Mumps, Rubella) was released, around 1972.
When a term for a nationality is considered offensive, it is usually paired with something bad that is not exclusive to them; for instance, French leave (military member absent without permission) and Dutch uncle (someone scolding, or speaking brusquely). That is, it is an offensive stereotype rather than a simple descriptor.
This is more in the nature of political commentary: when individuals come from a powerful nation, there is little position to be gained by their taking offense at the use of their name. In other words, oppressed does not usually modify German.
Would you have the same question regarding a common parasite hosted by children, the German roundworm?