I agree with this rule, but definitely not a lot of what's being said in here.
DNI lists absolutely open people up to harassment and bullying, and I am so glad that most of the people in here who have used DNI lists haven't experienced this, but that doesn't mean it doesn't happen. The fact that staff has made this decision alone tells me it probably has already happened many times on CS, and I know it has on other sites as well.
However, I strongly disagree that DNI lists including certain fandoms are merely "petty".
I shouldn't need to go into detail about this, but there are any number of reasons that seeing content from a certain piece of media or content creator - whether it's part of the conversation or not - could be triggering to someone. Even if someone doesn't know that a creator they're supporting is a deplorable person, it doesn't mean that it can't be upsetting or triggering to see their face plastered on someone's signature and avatar as they slide into your inbox.
I still agree with the ban on DNI lists, but please, be a bit more open-minded instead of writing people with genuine concerns off as being petty.