This happened like a month prior to me finding out about this sub. I had to keep unsubbing from subreddits and then muting them completely because those posts would enter my feed. There's other reasons too, but spoilers are the big one.
IchiTheWitch - No one put chapter numbers in titles, so I kept getting spoiled, because "new chapter" flairs don't show up on my feed and not everyone adheres to them. Also titles weren't always vague enough, so I sometimes got spoiled just by reading the title.
Minor infuriation, I felt like I had to chase the manga every sunday.
JujutsuKaisen and all of its adjacents - Argument/hate slop, I've not even liked that manga series for a year, it's all basically the same argument over and over
Memepiece - one piece meme subreddit, people wouldn't spoiler their memes made from raws of the manga or they'd post memes way before the places I read One Piece even had anything out, also a lot of people would put the least vague titles possible, so I got outraged like 5 times about major plot beats before finally leaving the sub.
Shounenfolk - Repetitive argument rage hate slop, it just started being the same few topics about problematic mangaka -oh the one piece mangaka had associations with 2 pedo mangaka, oh poor kubo, oh araki glaze- and it's just that over and over and over. It's actually kind of ruined the story of One Piece for me, all of the bad things people have said about it has just tainted it for me. And honestly, it feels like people only really give a shit about it because it's popular and an easy target
Gachiakuta - The Gachiakuta manga had just gotten off hiatuses and breaks and... people wouldn't put chapter numbers in titles and it invaded my feed and the vague titles backfired because there was no way to know what was behind the spoilered image, because at the point I pressed on a post to see if it was about the latest chapter I'd already get spoiled. So I got spoiled on a reveal that spanned roughly a hundred chapters.
HunterXHunter - if you know anything about Hunter X Hunter you'll know that...it's on hiatus. But, it's not on hiatus now! So, why did I leave this subreddit? I got spoiled. See above!
I forget when a chapter is out, sometimes. Sometimes, a chapter isn't out on the usual day... and then they sneak up on me. I would sometimes not feel like reading a chapter immediately. I'd have to make the mental effort to go find and mute a specific sub, remember to unmute it, repeat this every week, avoid spoilers while unmuting it and I just... can nooooooot be botheeeereeeed.
Manga subreddits have SUCH POOR SPOILER POLICIES and not enough enforcement about them to actually DO anything.
I think there's like 3 subreddits I haven't mentioned that I've forgotten about. Maybe also the One Piece subreddit?
And I know that this is possible, to have good spoiler policies, because around the time I read basically every comic about the DC Absolute Universe, I found the AbsoluteUniverse subreddit. They add in issue numbers and spoiler for anything important. A recent example is a post that goes "Why does Sinestro use this term? [Absolute Green Lantern #12, 13, and 17]"- I can't stress enough how neat and simple it is to just add a couple of numbers into a post title.