A friend of mine whose server I help out on decided recently to set up a NSFW channel for hentai. He allowed anyone who asked inside.
Within days, he received a warning from Discord about allowing minors into NSFW channels. He opted to delete the NSFW channel instead.
Which brings me to my suggestion: Why not require Nitro for access to NSFW channels? There is no other way to prove someone isn't a minor. It's quick and easy for minors who want to look at adult content to click through the warning message. Having a payment method provides actual proof.
During the Nitro signup process, Discord can also ask for the birthday of "the account holder" in the event that parents are buying the subscription.
I've heard of some servers doing scanned driver's licenses to access NSFW channels, but sending out that information to random anonymous strangers on the Internet can be dangerous.
I'd like to know what Discord and others here think about my suggestion.