OFAC's General License D-1 provides Slack with exemptions to offer their services even to users based in Iran:

(a) Effective February 7, 2014, to the extent that such transactions are not exempt from the prohibitions of the Iranian Transactions and Sanctions Regulations, 31 C.F.R. Part 560 ("ITSR"), and subject to the restrictions set forth in paragraph (b), the following transactions are authorized: (1) Fee-based services. The exportation or reexportation, directly or indirectly, from the United States or by a U.S. person, wherever located, to Iran of fee-based services incident to the exchange of personal communications over the Intemet, such as instant messaging, chat and email, social networking, sharing of photos and movies, web browsing, and blogging.

https://www.treasury.gov/resource-center/sanctions/Programs/...