First, I'm not really a fan of the way the OP in this thread was worded. It seems clear you knew we have a Slack already, and went on to imply that it was just ridiculous that we don't use Discord. Well, we have a lot of reasons for using Slack, some good, some neutral, but mainly we already use Slack so you should have to give justification why we switch.
All new features start at -100 points. The entire community is on Slack right now - you want us all to move so you don't have to download a new app? The points in "favor" of Discord, apart from history (and history isn't exactly awesome on Discord) seem to be more like feature-parity. "Well, you can do $thing_you_like on Discord, too, except worse, with a bot, or a shitty interface (or both)." What are the advantages to moving? Because I don't see any. History is like +20 points, so you're still at -80.
I don't see why this is even a discussion. Slack has worked fine for years. We've had our share of malicious users - about one every couple months, from what I hear - and we just ban them. It hasn't been an issue. The user privileges on Discord are worse - you have to be a member of every public channel on the server, you can't create new public channels as a user, and if you want a channel that people aren't autojoined to (thinking of something like #exception-thrown on Slack) you have to write a bot to add/remove roles from people.
Finally, and in conclusion, Screeps is a land of contrasts.
No, but seriously, I do have one final point. There are many programming communities that use Slack - I'm a member of a couple of them. There are no programming communities that use Discord. Screeps is a game about programming. The sort of person that would play Screeps, but doesn't know how to use Slack and is unwilling to, seems like the kind of person that won't like Screeps for very long anyway. Or, if they do get into it, they'll learn to write code ... and then they'll probably join a slack community ... and now they have Slack open anyway.
Don't uproot an entire community (or stir everything up) because one guy is whining about having to open a second app. What the actual hell.