In a side note that someone else already made: it is interesting to see that many companies that uses JIRA also uses Slack but the noise/complaint/mentions comparing when Slack is down is way different. I barely saw people complaning.
Slack is generally much more critical than JIRA in order to keep working.
You can do without JIRA for a week or two as long as managers understand and you all have a good concept of what work needed doing anyway. Then it starts getting dicey unless someone becomes a human JIRA to connect temporary manual bug tracking systems with everyone involved.
Other IM platforms wouldn't solve that just by existing. Sure, in principle one could set up such channels elsewhere, but that takes time, and the communication about it takes considerably more time.
Then if it does go down, you don't have to waste the first day arguing about the plan.