Yes but No. For most people writing this kind of mail should not be a problem. However, for many different reasons it can become difficult to write such things: This mail is some kind of formal letter and alters a contract. Let's imagine someone with a learning disability, they may be able to sign up for a online service, as they have done it many times. Writing a formal letter they may not have done many times and they cannot map past experiences the same way as a neurotypical person.
Depending on the local law this may be a reason why forced arbitration has to be opt-in: Typically the law should protect the weaker party. As the barrier for writing this letter is higher than the sign-up process, there is an argument that the chosen opt-out process of discord is targeted against some of their weakest customers.
@[email protected] already mentioned one. And it does not really matter what the can do specifically to you. It matters what they can do and that you have no control. If you want to know what people can do with just your username look at this project: https://github.com/sherlock-project/sherlock. Now imagine what someone with more data can do.