My input:
Discord is the way to go in my opinion, the first thing I did when I visited the Strapi website was ctrl+f for any discord server and was sad to see there didn’t exist any. Today I got pinged for the strapi conference on Slack and found the discord link and talked to DMehaffy about it being a better solution than Slack.
Discord saves all the message history unlike Slack which deletes everything after X amount of messages, Discord is easier to use, an easier UI/UX for the eyes, many customizable options like moderation discord bots or a ticket system like how the TypeScript discord handles several help channels (I saw a screenshot of that bot above here too). The role system makes you ping everyone with a certain role if you need help with for example @react ( I think Slack has this too actually), Discord is moving from gaming purposes to a more professional solution so expect more formal new features and so much more.
You can even use BetterDiscord which makes your Discord run with custom scripts, but I heard that’s against ToS so its bannable so use with caution.
Also I read here most of the reasons against Discord is that it’s hard to search/find anything and I absolutely disagree. It’s so much easier to find specific stuff, you can ctrl+f in any channel, type the name of the person who sent a message and say if it needs to include any image or link they have posted.