In general, re-implementing a software product from open specifications has been protected by the courts. There are numerous examples, from the original IBM BIOS to Microsoft BASIC to PostGres and BSD Unix. That said, the Google case is casting a shadow on programatic APIs, but 'look and feel' are essentially fair game. What are protected and protectable are Trademarks. So using the word 'Slack' in the name or anythi…
So calling something "Open Slack" would almost assuredly not be fair game... while claiming that the product is "mostly compatible with the Slack API" would probably be fine, especially since it almost self-evidently implies that the product is not Slack.
In any case, best to tread lightly with the use of their name at all, and certainly don't use it for marketing or promotion. Unlike patents and copyright, the only way to obtain and keep a trademark is to use it and defend it, so companies are rightfully aggressive in that regard.