It seems to me that every time people reference the slippery slope fallacy, the people claiming slippery slope get proven right 10 years down the line. Even in this case, banning gas in new buildings implies that gas will be banned in the future!
It makes no sense to call the slippery slope a fallacy. Some things are a slippery slope, others are not. But it’s not fundamentally impossible to have a slippery slope.