I don't see what your definition of 'political taboo', which seems to be related to top-down restrictions on speech or behavior, has to do James Watson's remarks.
There are few explicit or implicit rules about making racist claims that don't incite violence/hatred in most western countries (unlike the example you gave in Iran), or if they are, Watson didn't seem to suffer much for 'breaching the taboo'. Watson was shunned by the public and lost some scientific prestige/status because he didn't provide any evidence for his huge claims.
An editorial in Nature said that his remarks were "beyond the pale" but expressed a wish that the tour had not been canceled so that Watson would have had to face his critics in person, encouraging scientific discussion on the matter.
https://en.wikipedia.org/wiki/James_Watson#Public_remarks_on...I think there's a big taboo against making huge claims that aren't supported by anything other than your own authority (such as Linus Pauling claiming that Vitamin C can cure cancer), and an even bigger taboo when those claims are explicitly ranking groups of humans on the basis of their genetics or even vaguely defined features like intelligence.
I find it interesting that you're spending so much time talking about the presence of this taboo but no time at all analyzing or evaluating the actual claims. Because if the claims are false, who cares if they're taboo? Are all taboos bad? Is it a good outcome if we get to a point where all countries have the same taboos?
> If we are slowly conquering cancer, which once seemed intractable, we could slowly conquer stupidity as well, but that requires knowing something about the subject first, instead of blindly trusting some faith.
First we need to have good definitions for intelligence or stupidity. I don't particularly like IQ as a proxy for overall intelligence but if you are defining it using IQ, scores are slowly improving at the population level with hispanics and blacks gaining on whites.
Future Cognitive Ability: US IQ Prediction until 2060 Based on NAEP
https://pmc.ncbi.nlm.nih.gov/articles/PMC4603674