How to handle a common misconception when writing a Master's thesis?
I agree that option 2. is not good. If you want to mention the misconception but avoid shaming anybody, you can write something like


  "While the belief in the 1980s and 1990 was … it was proven in […] that in fact…"


That way, no one is blamed. If, however, you feel that you need to explicitly mention that still some people follow the old belief, you should back this up by citations and I don't see a way around option 3.