I'm surprised to hear this falsehood being repeated here. There is nothing inherently fallacious about slippery slope arguments. Like almost all rhetorical tactics they can be used fallaciously. The fact that slippery slope is not inherently fallacious is very well documented in the academic literature on rhetoric.

https://en.wikipedia.org/wiki/Slippery_slope

http://www.csun.edu/~dgw61315/fallacies.html