>>282045528 (OP)
Anime is a particularly bad medium for spoilers because of a combination of several factors.
One, most anime are adaptations of other stories. Manga, LNs, games, etc. There's very few original anime stories being made, so odds are whenever you start watching a new show, the story it's following is probably already several months if not years old, possibly even already finished.
Two, at least in the non-Japanese world, most people have to watch anime online, one episode at a time as they get released every week. Having to watch anime in that way at that pace naturally leads people to want to seek out online communities to discuss each episode after they watch it each week, which will unfortunately lead them into the same discussion spaces as fans of the original story. Speaking of which...
Three, fans of the original story. Odds are, lots of the people watching the anime version of a story will be fans of the original story. They want to discuss the adaptation too, but they already know the story so odds are they're going to mention things that haven't happened in the anime yet. They aren't deliberately trying to spoil things for anime-onlys, they just aren't thinking about them. That said, plenty of original manga/LN/whatever story fans are spiteful, petty elitists who look down on people who only get into the series because of the anime, and so spoil people on purpose to try and ruin their enjoyment of the story as punishment for not already being fans of it's source material. It seems odd to me to like a story, but want to ruin as many other peoples ability to enjoy it as possible, but well these people became friendless recluses on the internet obsessed with manga and LNs for a reason.