The permissions models for the two are wildly different because of the different use cases. Discord has a lot more by way of moderating and exposing different defined subsets of your server to different admin defined user groups. Slack has opt in on a per channel basis per user and unlike discord channels, any user can just create one rather than being these things admins have to set out and organise ahead of time. Slack has pinning as a "anyone can pin" default, the thinking being users pin stuff for theirs or others future reference, discord ties it to same permission as deleting other user's messages, thinking pinning is a moderator activity for making announcements.
Server admins control users on slack on the other hand. Your server admin can deactivate your entire account, DMs are tied to a server and your server admin can read them. It's a business tool, the expectation is employers have access to your business communications. The same on discord would be a massive privacy violation. But now as a business, how do you ensure that users aren't sharing passwords in chat, that your departed employee doesnt have a ton of company secrets in their dms etc
Server admins control users on slack on the other hand. Your server admin can deactivate your entire account, DMs are tied to a server and your server admin can read them. It's a business tool, the expectation is employers have access to your business communications. The same on discord would be a massive privacy violation. But now as a business, how do you ensure that users aren't sharing passwords in chat, that your departed employee doesnt have a ton of company secrets in their dms etc