Soil is alive, and plants have various direct and indirect symbiotic relationships with things in the soil.

Application and especially misapplication of various treatments (add fertilizer and fungicides to your list) can easily do significant harm to the soil ecology, which in turn causes get need for more or more intense treatments.

The easy answer is “never” the better answer is “smarter”.

Well it kills some living organism. These don't develop. It has a result on environnement including soil. Now it's it measurable everywhere? No.

There are a couple research papers and video documentaries that compare on the same area, sometime same hill, the difference in soil: one was marking as the only difference was treatment or zero treatment. Difference was around 5 inches / 15 cm higher soil in non treated parcel...

Now this is exactly the kind of question where everyone is going to find examples that suit their personal view, even with studies backing their point. It so much depends on region and soil, treatment, measurability of molecule effects etc...

Yes