When it comes to co-op games of this nature , I am of the mind that it's on the Vtuber to prevent any spoilers. So hiding chat is a MUST.
Personally I like being spoiled , I like to have the knowledge , I was watching Matsuri pov and not knowing who's the killer was kinda counter intuitive to me , perhaps because I watch a shitton of Trouble In Terrorist Town in multi-pov , with perfect information.
I understand that some people don't like it. It's a matter of perspective which way is better.

As for chat culture

I hate rules , any extra rules other than the very essential ones , put me off from participating at all.
No spoilers rule and banning for spoilers, sounds like the most stupid reason to ban a person.
It shows your own inability to deal with the situation on macro level, so you have to take it out on the dude that is emotionally invested in your win ,perhaps some kid that doesn't know better.

As for the "peko" and "pettan" thing, I saw it yesterday , cringe as fuck.
That's why I am against sexually suggestive content , Korone doesn't do that , she does yubi yubi and her chat spams yubi yubi , not "peko" and "pettan". Which is considerably better.
You reap what you saw, it's on them partially.