I'm guessing that you're being downvoted because there's a lot more to consider. I agree that it doesn't take much hardware these days (most single-board computers would work perfectly well) to service <1k simultaneous chat users with efficient server-side software (e.g. UnrealIRCd or ejabberd). However, to make it as reliable as Slack (99.99% monthly uptime is their SLA) for the price they offer it ( https://www.slack.com/plans ) would likely take considerable engineering effort. Sure, you could set it up, toss it in a closet, and it might have 100% uptime for a year...until it doesn't. If chat is business-critical, there are chat companies that have profit motive to deliver a good service. If chat is a nice-to-have at a company (and you e.g. don't have to worry about data retention laws / compliance stuff), maybe it's fine to run it on an rPi / t2.micro (free) AWS instance.

Luckily, there are a ton of great free and paid options out there these days!