I rhetorically agree it's not a good argument, but its use as a cautionary metaphor predates its formalization as a logical fallacy. It's summoning is not proof in and of itself (i.e. the 1st amendment). It suggests a concern rather than demonstrates. It's lazy, and a good habit to rid oneself of. But its presence does not invalidate the argument.

Yes, it does. The problem with the slippery slope is that the slope itself is not argued for. You haven’t shown the direct, inescapable causal connection between the current action and the perceived very negative future outcome. You’ve just stated/assumed it. That’s what the fallacy is.