This is another strong case for open source software development along with open, publicly available, standards and protocols.
This isn't about Slack, it isn't about US sanctions law. It is about any private entity creating closed sourced, remotely hosted software. Everyone knows that APIs can be pulled at any time (Twitter, Facebook, etc.) Spying can be mandated whether it is the US government, China, or anyone else.
As end users it is never clear what is going on behind the curtain. It is never certain the product will be here tomorrow - whatever the reason. No one would build a high rise where all of the elevators could suddenly vanish, yet we are increasingly putting the critical pieces of our businesses and our lives in the hands of entities that only need to flick a switch and everything instantly vanishes.