So the decking that was not on the ground rotted, but not the joists that were actually on the ground.
Is that what you're saying?
Ron

There is also some rot on the end that we can readily view now. It is in the support 2 X 6 that sits on the ground. We will see when we start removing the decking how much rot is in the support beams. I'm a female and my intuition told me it was NOT a good thing to place even treated wood directly in contact with the ground. So, I guess it wouldn't be defective wood since it is both support and decking that has rotted in six years. My question to my husband was... what to do with the support beams once we check them? The grade of our yard puts the deck right at the back door. We would have to dig out the existing support system and start over. GGRRR!

When CCA lumber was removed from the general market(It is still provided for marine/some commercial applications) much of the treated lumber that filled the void had to be utilized much differently.

There are about six different recipes for pressure treating wood---none of which equal the longevity and ease of use as CCA.

Now, I see HD is stocking a new brand that actually looks like untreated wood---called Yellow Wood or something similar. I can't wait to see how quickly it deteriorates.

Whatever you use for the decking has to be installed over construction lumber---and the approved exterior construction lumber is pressure treated. So, what ever is rotter needs to be replaced ----and needs to be installed properly. That process includes things like elevated post bases(metal bases fastened to the concrete that elevate the wood a couple inches off the concrete), or setting the posts in concrete(not the best idea since that also allows rot).

It seems from your description that excavation of the soil, installing concrete pilings to mount the deck framing on and rebuilding the deck framing with some space between the framing and the soil is the best long term solution.

If at all possible you want to avoid wood being in direct contact with the ground. If you have to do that, you need pressure treated wood rated for ground contact, which is hard (impossible?) to find at the big box stores. At the very least you should consider digging out some of the ground and replacing it with stone so water doesn't pool around the timbers.

Ideally you should dig out enough to put in concrete piers and get the wood structure away from the ground.

Since the deck is so low and damaged, rip it out. Put in a stone patio and be done with the constant maintenance a deck needs.
Ron

second the stone patio idea
we should have done that instead of getting our wooden deck replaced with wood vs doing a stone patio--even though deck was elevated and partially under porch roof there was too much water/sun deterioration even when the wood was treated--