Earlier quoted context omitted.
There was indeed a slippery slope from "2 weeks to flatten the curve" to extended lockdowns. I don't think those policies necessarily were a net benefit, either, particularly in the case of mask mandates for children, which weren't shown as effective enough to warrant damaging the mental health of children. https://assets.publishing.service.gov.uk/government/uploads/...
Something taking longer than expected doesn't mean it's a slippery slope. Whether they were effective or not is orthogonal to whether the slippery slope argument is a fallacy.
> Whether they were effective or not is orthogonal to whether the slippery slope argument is a fallacy.
That is wrong.
What you mentioned is the very definition of a slippery slope, where the conditional relationships of the hypothetical syllogism do not hold.