I figured anti-polyamory propaganda did not need annotations on LessWrong. It's telling that all but one reply took it as something which needs to be suppressed/counterargued, despite me calling it propaganda and saying it was interesting as an example of psychological tricks people pull.

If a trick is trivially seen, there likely no update made by seeing the trick in action and I don't see the argument for the value of seeing it in action. To the extend you claim you saw new tricks that you weren't aware of in the past that does raise the question of how you conceptualize those newly seen tricks.

I take the negative feedback as meaning content people are politically sensitive to is not welcome, even if it's rationality relevant (resistance to manipulation tricks)

Political sensitivity has nothing to do with my assessment.

You could label any bad source on the internet rationality relevant by saying that it serves to see bad reasoning in action. You haven't provided any argument why this particular piece of propaganda is more worthy of attention than other pieces of propaganda.

Apart from that I'm doubtful that the mechanism you propose actually leads to resistance to manipulation tricks. Adopting new habits is hard.

If the article lead you to see manipulation attempt at content that supports your own position that you previously haven't seen that would interesting information to talk about. Till now I haven't seen that the video had that effect on you and even less that the video has that effect on other potential viewers.