You should have corrected your wording here. It's "
NSFW channel" and not "
NSFW server." The way it's worded made it sound like you were talking about two separate servers.
In Rule 6 of Leaflit's Discord server, it clearly implies that minors are allowed, and despite that, it's still treated as "
an adult server."

Now that is a bit strange. If the server should be treated like an adult server, then why are minors allowed entry in the first place?
Now, for the NSFW channel, there appears to be a form of gatekeeping. Users need to have the "
Adults" role in order to access the porn channel. I am unsure what exact method and process the moderators use to confirm someone requesting the role is actually an adult or not.

Them being present in the general chat isn't an issue; instead, it's them being in the server in the first place. Especially when the rules clearly state the server is treated as an adult server. The people in the chats are talking about adult topics as well, even if they are not full posting images or videos of porn, the adult topics (like sexual stuff) being discussed is very inappropriate for minors to be engaging with. For example, a minor talking with adult strangers about femboys or porn is a major red flag.
This headache could all be avoided if they clearly stated minors aren't allowed in the server. Will this prevent every single potential minor from joining? No, of course not, but it will lessen the number of minors from joining. Since they would be banned if caught or leave of their own accord. And yes, of course minors can lie about their age, but in that context it can be argued the server isn't directly at fault for that deception. It's a hard issue to tackle, but it's better to attempt to tackle it instead of basically having a sign that says, "
Minors are welcome here! But we talk about adult topics, including sexual stuff. What could go wrong?!"
Having an NSFW porn channel and allowing minors in the server at the same time is
NEVER a good idea. Even if the NSFW channel requires some sort of verification, in this context it would be a role, which is indeed better than no role verification, but it's still a bad idea overall. Either ban minors and have an NSFW porn channel and allow the discussion of adult topics, or allow minors but don't have a porn channel and crack down on adult topics or segregate them.
Better yet, don't allow minors and don't have a porn channel at fucking all. It's that simple, Leaflit is the boss. If she says no NSFW channel, then there will be no NSFW channel. I don't see the point of such a channel. Shouldn't a Discord server for a VTuber be a place to discuss them and not to goon like a porn addict? Particularly since Leaflit is pandering to a right-wing and religious audience. The presence of a porn channel is so baffling.