I'm another who wonders why you didn't just ask him. I'm not sure if you can see messages once banned - I know you can read some channels even if you're unable to post in some areas - but he could show the general of discord if you don't know about it.
Discord is huge, it would be like a parent of teens in the '90s not knowing what an AOL chatroom or MSN messenger is, or like a pp said, like Snapchat ot TikTok now. Even without either you or the teens using them, you need to take a highly hands off approach to teen internet access to not know the basics of those. I sorta get why some parents prefer that route, but I think the risks of it are far higher than the risks of access at a friend's house or 'they'll just get around it' would like to think. I mean, why would they talk to you about something you know nothing about and have shown no previous interest in to the point you don't even know what it is?
Being above the minimum legal age requirement doesn't mean a child is old enough, many places online don't have any just like books don't have any. It's possible to have open communication and strong restrictions. I've literally had a teen ask me to maintain higher restrictions because he found the access he had to internet browsers and apps "too tempting" (particularly for violent graphic novels that apps rated as 13+, but, as I said, with books that's subjective) when he was 14.
My father was in a company that had WFH from the late '80s, I had open internet access from the age of 7, just as I had free access to miles of woods at a similar age, and free access to alcohol from my teen years. Many parents are a bit more cautious about all those these days because shit happened. I'm more rigid on internet access than anyone I know - to me internet access is like outside access, you do it together for a while, then they take gradual steps towards independence that might require some monitoring, back steps, and restrictions. We also have a ton of open conversations about it because I lived the risks of expecting a child to manage their own access, I constantly hear the drama from others about what's going on with their kids internet access and I'm actively involved in their internet lives like the rest of it. My teens like showing me their hobbies, on and offline, so I know what Discord groups my son is part of because I ask him about it and we talk. When I talk about it less, so does he, it's up to parents to openly discuss it, not put that on teens.