It's a book review thread with a "Spoilers!" warning, that should more or less set your expectations for what the reviews are allowed to do
And I think there's a wide range between spoiler-free reviews and "storyline breakdowns". The kind of review I like reading most is a review that discusses a novel's story in a meaningful way, ponders what it has to say and how it stands in relation to other things, real or not - and while that usually necessitates spoilers, it's a far cry from a mere plot summary.
My understanding is that in the past, there were often two review threads opened, one with a spoiler warning and one without. The spoiler-free ones seem to have died away at some point - presumably because the fragmentation was undesirable and folks found the spoiler-free format too stifling. Remember also that discussion is the point of a discussion board, posting isn't a one-way street so it's not just about the reviews but also about the constraints for the dialog - posting a spoiler-free review is one thing, disagreeing with it without referencing plot details to build an argument is often harder. So while of course it's possible that the audience and preferences have shifted now, it also seems possible we're repeating a conversation that has already run its course.