Earlier quoted context omitted.
In any case, it probably is not worth likely legal costs and effort should the other party take legal action. It is one thing to theorize about legal implications, quite another to risk livelihood etc to put it to test. Defending against action takes time and money which amounts to loss in many cases even when you are successful in court.
There is almost zero chance slack would win this case and I doubt they would even try. Adblockers inject code into sites. So do password keepers. If sites could sue adblockers because it breaks their terms of service, don't you think they would? You are not bound by the slack terms of service except in the scope of your slack account or an API connection of some type. Someone else can use my software all day long to…
Being "right", sometimes, is not worth the fight.