Rowan Atkinson gives some nice examples here:
https://www.youtube.com/watch?v=xUezfuy8Qpc
I'd say rather than there being *a risk* of descending too low on that slippery slope, we already have plenty of evidence this already happened. I'm from Poland and recently a party in ruling coalition proposed criminalizing hate speech on the Internet. Fortunately, precisely the slippery slope rhetoric, supported by evidence from countries like UK, seems to be enough of a backlash to stop politicians from following such ideas.