Saudi Arabia is a large investor in Slack. Probably doesn't mean much here, but you never know.
United States sanctions: https://www.treasury.gov/resource-center/sanctions/Programs/...
Is Twitter, Google, Microsoft behaving the same here?
P.S. On a side note, if these sanctions are really about 'punishing the regime', blocking mediums for Iranians to speak on isn't a way to do it, but what do I know.