What criteria do you use to evaluate that intervention? Which really means, how do you begin to estimate the actual amount of harm reduction that it provides?
Are there lots of cases where someone tried to overdose but was saved by it 'only' being 16 grams of paracetamol?
The number of deaths due to paracetamol poisoning fell by 22% in the year after the legislation. There was no significant increase in suicide deaths by other means. We see similar results with interventions like reducing the availability of highly toxic pesticides or installing barriers on bridges.
Most suicidality is impulsive and transient, so simple interventions to reduce access to means of suicide can have very significant results. There's very little you can do to prevent a sufficiently determined person from dying by suicide, but most people who die by suicide aren't strongly motivated to die - they're just briefly overwhelmed by life.
I can only speak from my experience, but at one point I took all the paracetamol I had (suicidal and pretty drunk, the taste of cheap red wine and paracetamol is... unpleasant to say the least). I ended up fine after a friend forced me to go to the hospital the next day. If I had more tablets around, I would have taken them too. So it likely wouldn't stop someone who planned to kill themselves, however with me anyway not having like 100 tablets around did help due to it being impulsive enough.
It might have been quite a big difference due to how long after the fact I went to the hospital, I took quite a lot but it had done little enough damage that some medicine in my arm over the course of about a day and having to avoid alcohol and paracetamol for a while was enough to fix it.
I imagine a lot of cases are quite similar to mine and it probably did make quite a big difference with me as I only had 2 boxes around and one wasn't completely full.
The grandparent of the post you're responding to provides stats showing that suicides/overdoses with OTC painkillers went down markedly after the change.
Are there lots of cases where someone tried to overdose but was saved by it 'only' being 16 grams of paracetamol?