I think there is a considerable cultural difference at play here, between the UK and US. The British public tends to be fairly don't-like-it, switch-it-off, on the whole, believing people should take personal responsibility for whatever "triggers" them (rightly or wrongly - I don't mean to dismiss your points). That's why, as per the article, the BBFC offer general non-spoiler warnings, then more detailed warnings (that can contain spoilers) for those who want to go online and check them out first (for mental health or any other reasons). I think this is great practice as far as cinemagoing is concerned, and the right balance has been struck, on the whole.

We've had uncensored public TV stations for years, whereas (I understand) censorship is still very much a thing in the US outside of subscription channels and streaming. I think that's also a factor in why we tend towards encouraging personal responsibility. However, the increasing prevalence of a certain kind of West Coast walking-on-eggshells attitude is creeping in here on streaming services (probably copied and pasted from their US versions), which tends to provoke more eye-rolls than anything. But I'm sure such an approach has its supporters here too, even if they are in a minority.

My point is this: As per the end of this article, my suggestion is that on streaming services, those that prefer not to have full-frame captions spoiling dramatic works (I suspect the majority, certainly in the UK) should have the ability to opt-out of such warnings. Therefore everyone wins. I don't think that is unreasonable, personally. :)