I switched from Slack to Discord back in 2017 and I can't imagine ever going back. Their free offering is better than what you get for $$$$ from Slack.

Slack is designed for small groups of people that all know and trust each other. That security model falls apart when you scale to large low-trust organizations. Discord was designed for strangers and offers far more granular controls.

They offer infinite search. Unlimited users. And it's free! Can't recommend it enough.