It's only a logical fallacy in the form if p then q; if q then r if then ... so on until the end point you're arguing towards. To say that history informs us that a particular hill easily becomes slippery is not a logical fallacy.
It's a fallacy whenever someone argues from it. Like the GP, they are literally arguing from a SSF and no one believes that's what it is. Beyond the pale.
" I'm very concerned that the US government will set a precedent for regulating what apps citizens are allowed to use."
Since US law is based on precedent, it is not unreasonable to say that allowing the government to legislate which apps are available to US persons could lead to creeping powers to regulate apps more broadly. All of post 9/11 security law is practically a national park of slippery slopes.
I see the confusion. I was referring to "this feels like a slope that could become very slippery." having any meaning outside of someone arguing from SSF.