It is trivial to find servers with adult topics (BDSM) targeted at minors. It is trivial to find servers that combine those topics with problematic age ranges (like BDSM-themed servers with no ID verification, 'ages 14-28 welcome'). It is trivial to find servers with minors openly selling "content".
Disboard isn't Discord but these things aren't even being remotely "hidden", it is these servers' sole 'purpose'.
Phone and mail networks in letter and spirit obey the law: they don't listen to people's private conversations and they give up complete user info on request by law enforcement. There is nothing more they can do.
Discord has built the capability to read every message, public or private, that any user sends. So they are ethically obligated to stop bad things happening on their platform. Whatsapp/Signal have built their platforms so they can't read user private messages, so they have no ethical obligations to stop bad things happening on their platform, beyond banning users in response to legal orders.
So they need to get rid of that capability
Why draw the line there? Why don't those platforms have an ethical obligation to build the features that would allow them to stop bad things happening on their platform? Especially if they knowingly developed the current implementation specifically to avoid ethical culpability?
So I am glad that software like Signal/Whatsapp exist that allow secure communication [1]. And I would take the harms causes by them being unmonitored rather than the harm of future dystopian governments. Due to how crypto works, I don't think there is much middle ground here.
[1] I would prefer open source, more community owned platforms take over than these two.
Phone and email aren't more private than Discord either. Arguably less. Difficult to get a phone these days without buying it on camera. And a phone company will give up all your messages.
Whatsapp/Signal are as close to (1) as possible by design, can't actually do (2) at all. Phone/Mail are somewhere in the middle, but quite close to (1). In most countries in the world, there is no mass recording of phone calls or mail, despite it trivially easy to do so. Moreover, due to long long historical legal precedent, I don't think phone/mail companies have any freedom to do things differently. They are pretty much constrained to do exactly what the government tells them to do.
Discord on the other hand, does not respect (1) at all. In fact, it very intentionally records and reads everything for profit. And it hands over any info requested by law enforcement. So, ethically, either they rewrite Discord to respect (1) or they should do (2). I don't think they are. As others have noted in this thread, it is trivial to find servers that are clearly criminal.
One might argue that it is impossible to do so because there are so many servers. My second political position is that if your public platform is so large that you can't effectively moderate it, that is not an excuse. You are culpable. Simply stop your platform from growing past the point where you can't effectively stop bad things happening. You don't have the right to profit while enabling bad things.
I do find it interesting that we hold these platforms liable but not the phone/pager/mail service. If this doctor had called this girl on her cell phone, no one would be mad at Verizon.
Part of the problem is most parents have no clue about social media/communication tools outside what they use. At my church, I gave presentation about Discord and it was shocking to see how clueless parents were.