17. No GI caustics
In the GI settings, turn off refractive GI caustics. You will clearly see that shadows get darker. No light is able to pass trough the transparent materials.
This is just a reminder to show you the effect of GI caustics. Turn the refractive GI caustics back on.
I usually don’t use reflective GI caustics, because they do more bad than good in most cases. Refractive ones are way more important as you can see from this example.