It depends how important suspense/mystery is to the story, and if you enjoy suspense/mystery.
If you enjoy suspense, and a story is elevated by suspense, then spoilers matter.
If you don’t care about suspense, then spoilers don’t matter, and you don’t appreciate when a story makes use of suspense (nothing wrong with this, people can like different things).
Yes because there is no point in finishing if you don’t have the surprise.
Yes, it does. It is part of the fun.
For me specifically, if spoilers hurt a book, it probably wasn’t worth reading in the first place. I love when authors demonstrate mastery of language and narration, and no amount of spoilers can overshadow the direct experience of witnessing it enacted.
If I’m into the story, yes. Feel like this kind of question doesn’t understand that a lot of stories only have going for them resolving one or two questions. Once you know the answer the rest of the story is so average it’s hard to care to finish. For example I read a lot of otome isekai (tl;dr romance novels where the heroine was sucked into a novel/game). Once you know the twist it’s just a subpar story.
That said if a story is kind of shitty and I’m considering dropping it, but I read a spoiler the twist is actually really good and puts the crappy parts in an interesting context I’ll keep reading. So case by case, reader by reader basis. I have stories I only finished because I read a spoiler, but I have just as many I never finished because I read a spoiler.
