Just spotted this as Slack has recently updated their API Terms of Service, though it may have been in there before this update?
Of course, faced with either jail time courtesy of the government or breaking Slack’s TOS, which do you think a developer would choose? Slack legal time just covering their arse with this clause.
9.1 Government Access: You will not knowingly allow or assist any government entities, law enforcement, or other organizations to conduct surveillance or obtain data using your access to the Slack API in order to avoid serving legal process directly on Slack. Any such use by you for law enforcement purposes is a breach of this API TOS.