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...