I see mine is doing it now too.
I noticed it was a very clean, mobile looking sort of layout. So, on a whim I forced the desktop version of he site. The layout went back to the old look, and was zoomable again.
In iOS 9, you tap-hold the refresh icon (circle/arrow) in the URL field for the Request Desktop Site option.
The setting seems to stick as long as you keep using that tab. If you open a new tab, it reverts to the Mobile version of the site, which can't be zoomed.
I suspect this a choice Google made, and not Apple's fault. If you search for something like "dog" the first results on the mobile version are interactive; two side scrolling image lists and a Show More button. This portion of the page would not work well if you zoomed into it.
On the desktop site, the same information is in the right sidebar, and is not interactive.