Click, done. — Several People Are Typing — The Official Slack Blog

Slack expands the scope of scenarios for which the message is the medium

"Imagine getting an expense report in Slack and simply tapping Approve or Deny to change its status. Instead of having to search your inbox or open three new browser tabs, you’d be able to complete your task straight from Slack and move on to what’s next.
Now you can do just that in Slack with Message Buttons. These buttons make Slack the place where you can click one button to get your work done, whether that’s posting a tweet, approving a new job posting, or filing your expense report."

Click, done. — Several People Are Typing — The Official Slack Blog