Well… in the first place, it can’t be an example of a slippery slope fallacy, since the possible slippery slope (at least what I presume you’re considering a slippery slope) isn’t employed as part of a chain of reasoning or argumentation.

A slippery slope fallacy is a chain of reasoning or argumentation that follows the form:

-A

-A slippery slopes to B

-Therefore B.

It’s a potentially reasonable bit of inductive reasoning, since it can be argued that there’s some likelihood of B, but since it does not prove B, it fails as deductive reasoning, which is exactly how and why it’s a fallacy.

And that’s not the way that Kant uses it anyway.

I presume that the thing you’re characterizing as a slippery slope is the concept of universalizability - that one should “act only according to that maxim whereby you can at the same time will that it should become a universal law.”

But that’s not saying or implying that it will be the case that if one acts in some way, others will as well, so it’s not a slippery slope at all. It’s more just a thought experiment - “in order to test your judgements of how to act, imagine how it would be if everyone else did the same.”

I’d say that Kant was wrong in a number of ways, but that wasn’t one of them.