During today's Slack downtime, we just communicated with each other use that old, reliable, uniquely human analog method.

If you're on the same network you can use multicast-dns (like what Chromecast uses)

Absolutely. ad-hoc networks, using sound/bluetooth as a transport. Connected to the same network in general. We have all these things available for us to use, but we haven't so far. (well, there is experiments, but no big application like Slack has actually built something solid and usable with it)