It's a matter of degree. CentOS infamously chose to refer to Red Hat as "a prominent North American Linux vendor" and completely deleted the term "Red Hat" from

It's a matter of degree. CentOS infamously chose to refer to Red Hat as "a prominent North American Linux vendor" and completely deleted the term "Red Hat" from everything out of an abundance of caution after receiving threats from Red Hat's lawyers, but that's probably overboard. It's generally allowed to use trademarks as "nominative uses", i.e., in direct reference to another's product that's not designed to create confusion. He would probably be fine to put in his README something like "An open implementation of the chat API provided by Slack". He definitely shouldn't use the term "Slack" in the name of the repo. Like most non-flagrant potential IP violations, no one can really say for sure until a court hears the case and makes a judgement as to whether the use is fair or unfair, or likely to cause confusion among consumers or not. IANAL.