Sounds like having a fallback pre-defined would be prudent if it's that important and you don't feel you could collectively extemporise something. "If Slack goes down, the plan is to use WhatsApp/Teams/Jeff's Matrix homeserver in his garage until service comes back. A list of group channels will be emailed if that happens."

Then if it does go down, you don't have to waste the first day arguing about the plan.