I don't think it's about probability. The Slippery Slope argument says that if we make small change A, then we will subsequently make larger change B, followed by a series of ever large changes C, D, E etc until we make extreme and horrible change Z.

This is usually a fallacy when used in an argument, because it ignores the fact that in most situations each step is a choice, and we can halt the progression wherever we want.

The exception where it is not a fallacy would be where, once the first step is made, there is no possibility of stopping or turning back. Examples of that in the real world are hard to imagine. An example from physics is where one crosses the event horizon of a black hole. If you do that, there is no turning back, and you are doomed to be spagghetified by the extreme gravitational forces nearer the centre of the black hole. You will feel perfectly fine after crossing the event horizon, and for some time after - everything seeming perfectly normal. But after a while you will be pulled towards the centre and exterminated. You cannot escape.

I can't think of a similar exception in politics or ethics. All the Slippery Slope arguments I have seen in those domains are nonsensical. But theoretically one can imagine that there might be a case where there is one.