This doesn't feel like a real question... Slack free tier is basically crappy Discord, limited message history, no voice channels, huddles are also behind the paying tiers. It is basically worse on all aspects unless you start paying

A nonprofit I help out just moved from Slack to Discord for a very simple reason: Slack pricing was too expensive, and as the amount of people increased, the price continues to climb. Discord is free

Most importantly, Slack limits the amount of message history you get to keep if you’re not paying. And the payment plans are per-user fees which quickly becomes non-viable for non-commercial use.

reliable message delivery, lol. slack drops messages silently. it is not fit for purpose.

The biggest one for me is that Discord will keep all history for free servers, whereas Slack only gives you access to 3 months iirc (and as of a year or two ago, has started permanently deleting older content).

For large communities, the very granular role-based permission system of Discord can be put to some good use, I don't think Slack has a trivially equivalent feature.

Dude, Slack deletes everything almost immediately unless you have a paid version which isn't cheap.