The fallacy as I understand is regarding the probability of the results of any one action, as the unsupported assertion is not how the slippery slope argumentation works since the premise is not any assertion but an action, and the action is not where the error lies but only in the asserted result which follows from the act, and it would be an error only if the probability of such a result is not likely. Here's the wiki regarding it:

Logic and critical thinking textbooks typically discuss slippery slope arguments as a form of fallacy but usually acknowledge that "slippery slope arguments can be good ones if the slope is real—that is, if there is good evidence that the consequences of the initial action are highly likely to occur. The strength of the argument depends on two factors. The first is the strength of each link in the causal chain; the argument cannot be stronger than its weakest link. The second is the number of links; the more links there are, the more likely it is that other factors could alter the consequences."

Now, is it a fallacy or a truism.