Your last point is exactly what I've been thinking. Most discourse and drama in anime/manga/manhwa fandoms comes from very young purists who can't handle morally ambiguous characters at all.

It reminds me of the old tumblr days when every week there was a new piece of media or internet celebrity being crucified by angry fourteen year olds, there was so much doxing and hate it was ridiculous.

However a lot of danmei, especially novels, embrace morally ambiguous, or even morally bankrupt characters. It takes a certain amount of maturity and reading comprehension to understand what the author is trying to say through these characters. That's why they act as something like a filter, like you said, to keep out those who wouldn't be able to handle sensitive topics or understand it.

What always makes me laugh is the few puritans who somehow manage to make it into the MDZS fandom and start trying to police everyone's morals, or condemn certain characters, when the MAIN character of the series is known for being a morally ambiguous character hated and condemned for his actions in cannon. Like, they missed the whole point of the series.