https://slack.com/security
I was also a bit surprised what they consider out of scope for their bug bounty program: https://hackerone.com/slack
They've sent bug reports with credit card data they've typed in during a phone call through a variety of insecure methods.
They've also written people's credit card info on sticky notes.
Trust me, the horror that is card data and a call center is scary.
I was also a bit surprised what they consider out of scope for their bug bounty program: https://hackerone.com/slack