[BjoernKW]

A lot.

As a consultant I work with multiple clients at a time. Sometimes I have to accommodate their tools (such as Slack) and processes but email and calendar still are the lowest common denominator for communicating and staying in touch.

I actually prefer traditional email and calendar because those are open, federated protocols that allow me to use my own infrastructure and tools rather than relying on the client's infrastructure.

Juggling multiple Slack (or similar) accounts is cumbersome and doesn't integrate well with other tools and external processes (integrating one Slack account into your process usually works pretty well but as soon as there's more than one account things get messy).