https://www.reddit.com/r/discordapp/comments/9p5uyp/latest_tos_update_revokes_your_right_to_sue/
The top stickied comment to that thread is a staff reply, if you want to check that out. The gist of it is that it'll apparently protect them against frivolous lawsuits that law firms get rich off of.
I had my worries about Discord and was reminded of what Stallman says about discord here https://stallman.org/discord.html. One of the pages he also links to there speaks about the possibility of discord selling their data in the future, even though they claim not to at the moment. Perhaps behind the scenes they're preparing to protect themselves for something like this down the road.
One thing to note is that new accounts can opt-out 30 days after creation. Hopefully they'll add a notice about this in the account creation process in the future, or even better a yes/no dialog to make it easier.
[Edit] Turns out Discord's CEO apparently has a shady past in which he sold his previous social platform to a Japanese company without any notice to users. The company then harvested all the data and shut down the platform, but not before being hit with a class action lawsuit.
Source: https://www.reddit.com/r/discordapp/comments/9peikv/in_light_of_the_tos_update_regarding_class_action/
[Edit] They changed it. The opt-out period is now 90 days and the arbitration clause now only applies to users within the US.
https://blog.discordapp.com/terms-of-service-feedback-and-changes-c58a530c7693