Im Just A Silly Little Guy

go to discord support ( https://support.discord.com )

sign in or sign up (you do not, and imo should not, use the name and email you use for discord proper. this is a different account; if you've got Firefox, you can use an email relay mask very easily, and the email will end up in your inbox but the site won't know your email from it)

"Submit a request" (at the top)

fill all the dropdowns (I did "Help & Support" -> "Technical Support" -> "Account Settings" -> "[OS type"). it won't let you proceed until you do

Subject line: something about age verification

description box: keep it civil, but be clear. you don't like this, you're not participating, and you're not giving them your money anymore (whether you had nitro or not is irrelevant, remember that you didn't use your actual discord login to get here and they have no way to verify).

some reasons possibly worth mentioning: the insecurity of databases (Discord's had multiple leaks; databases being hacked isn't possible to prevent afaik, it's a matter of mitigation), the dangers of putting one's govt id on such a database, the technical problems people are already experiencing where it's already been established, how it will disrupt communities

be very clear that this is going to cost them money. "I won't use your service anymore" is a common threat ("I'm never shopping here again!") - you need to make them feel that they are losing money just by considering it, and it will get much worse if it's implemented.