I truly despise it. I have a dog whistle right here. Go ahead, blow on it. I assure you no one will hear.
Something that is hard to defend against and intended solely to shut down the speech of other people with whom the (leftist) utterer of the phrase disagrees. Speak out about some policy you wish to advance? Ha! That's a dogwhistle to racists/misogynists/homophobes and you can't prove otherwise!
Perhaps dog whistles is about a special language that two groups understand but the middle that does not.
Group A) The group that created the language in the first place.
Group B) The group that has experience that was negative due to "special languages" such as they got hurt, and thus they are hypervigilant looking for all special languages even if this creates false positives all the forking time.
And project exists higher in both Group A and Group B compared to the middle who does not understand nor care what is happening with the dog whistles.
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Oh trying to educate people about dog whistles without an "experience" component that grounds a person is often ineffective and sometimes even counter productive for it just enlarges the amount of people who know the special language in the first place.
So I repeat "Perhaps it is not about proof ", it is hypervigilance gone awry hoping to prevent the scariest things you can imagine for some things can't be repaired when broken. "Proof" is not helpful in such a situation for proof is after the fact (after the thing is broken.)
Yet a hypervigilance mindset creates its own form of problems.