I bought a bike used which came with a worn pair of WTB All-Terrain tires and I would like to order a replacement pair of tires, but it looks like I can only find them in 32mm+. I wanted to know why the sidewall of these tires say they're sized as 30/32 and if maybe the size of the tire changed over time? Is it possible these tires can be both 30/32? Or maybe I'm reading it wrong?
The 30/32 is WTB's old slightly gimmicky sizing system where the two numbers are tire casing width and tread width at widest point in millimeters. WTB is not using it any more, but it's still explained at Sheldon Brown's web pages. The rim diameter is the same for different tire widths, so any tire with same diameter and width somewhere close will fit.
I believe 32 is the tire width, and is measured in millimiters (mm) not centimeters. On their website I see this model is described as 30TPI (threads per inch, describing the structure of the fabric under the rubber) and 32 width. Maybe that's why you see 30/32 on your old tires' side wall. Or perhaps the manufacturer specified a width range of 30 to 32 mm depending on the rim width you install the tire on; the tire will be effectively wider if installed on a wider rim.
Here's a guide for choosing the correct tire size: https://www.rei.com/learn/expert-advice/bike-tires.html I'm sure there are also plenty of questions on this website as well regarding the topic.