Discord is focused on large groups. E2EE doesn't work in this case. Group management overhead traffic is too high and too unreliable, and a bad actor could just join the group under a pseudonym to log messages. Discord isn't E2EE for the same reason Hacker News isn't.
For large public servers it would be pointless. But for DMs and small private servers, it would be meaningful. Most people in DMs and small private servers would not appreciate their messages being publicly accessible like HN comments are.
