A disclaimer which says “this will void your warranty” is perfectly legal. Breaching the limitations of a warranty is not illegal. In fact in some jurisdictions, the limitations of a warranty are themselves not enforceable (in the USA under the Magnuson-Moss Warranty Act)

Inducing a breach of contract is a tort [1]. It does not apply in all cases where someone is induced to break a contract but in this case where the extension author has knowledge of the Slack terms of service and the extension targets Slack only, it almost certainly does apply. This argument was used in the case of Blizzard vs Bossland [2].

[1] http://www.oxfordscholarship.com/view/10.1093/acprof:oso/978...

[2] https://www.bristows.com/news-and-publications/articles/bris...

How many companies do you think have successfully sued ad blockers?

There's far more of a financial incentive there.

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 break their terms of service, but that doesn't make me liable. I didn't agree to anything.

People need to realize that terms of service are a civil contract. It's not "illegal" to break them. Especially if you didn't agree to the terms.

Has anyone (with tons of money to spend) ever made a suggestion to you that they might sue you? It is pretty stressful. I do not recommend it to anyone.

This is the difference between fantasizing about the likelihood of outcomes when you have no skin in the game and being on the other side of a well funded adversary that is threatening a lawsuit. Put yourself in the POV of the one with skin in the game and consider that it may take years and insane expense (time/money/energy) to prevail against even a poorly founded lawsuit.

Being "right", sometimes, is not worth the fight.