The experiment where they change the color of the submit button? What should cause me to care about that?

And what does opt-in even look like? No matter whether you want to "participate in the experiment" the submit button still needs to be some color for you, which is the only part of the "experiment" with any direct effect on you.

The concern with psychological experiments isn't that they're collecting data. That's a different bailiwick. The major issue with psychological experiments is that they may have significant direct psychological consequences. If you show people only news stories about mass shootings and conflict it may cause them to become violent or suicidal -- which has nothing to do with whether you collect data on it or what you do with it afterwards. The experiment itself is the harm.

Which means we would need some kind of principled and efficient way of distinguishing those kinds of "real" experiments from just measuring what happens when you make a subtle adjustment to a context menu.