If I were a developer of Slack, I would be very tempted to disable that OS functionality (similar to how Sublime Text does not seem to utilize this OS feature).
Turn it off at the OS level if you don't want it. Don't make a decision that makes everyone else who uses the OS frustrated because functionality only exists in a subset of applications.
Which leaves me juggling there on my keyboard, trying to delete one, move my cursor and add a second dash before submitting.
System Preferences / Keyboard / Text / Use smart quotes and dashes = off
Source: I was a teen during the Desktop Publishing craze