I'd like to add... the key test for trademarks is whether or not the use will cause confusion in the marketplace as to the origin of the product. So calling so

I'd like to add... the key test for trademarks is whether or not the use will cause confusion in the marketplace as to the origin of the product. 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.