They removed the arsenic from treated wood in 1993. The pier and stairs/walkway that goes down the cliff to the river was built in 1991, same time as the house. Ground contact, stays wet, no maintenance - no rot or termites at all.
If I built the same thing today out of treated lumber it would not last 5 years.
You can still get the arsenic treated (CCA) wood in timber sizes. (At least you could a few years ago.) Can’t get it in dimensional sizes. Fear of it being used where kids can chew on it. You can thank 60 minutes for that one. Same reason the YJ was lower stock from the factory, 60 minutes expose on dangerous 4x4 vehicles.
There were a few years that the wood treatment process was really terrible. The chemicals used for treatment would rot any steel in direct contact. You had to use coated fasteners and hope there were no scratches in the coating, or use stainless.
I used Trex about 20 years ago to make a shade structure, boards were spaced about 2” apart. That composite wood is still in excellent shape. But it is about 10’ above ground in the SE Texas climate. It was all nominal 2x4 material. Heavy as heck when I installed it.
Other treatment types
CCA - Chromated Copper Arsenate (man, I miss that wood!)
MCQ - Micronized Copper Quaternary
MCA - Micronized Copper Azole (most common in Texas)
CA - Copper Azole (corrosive to uncoated fasteners)
ACQ - Alkaline Copper Quarternary (corrosive to uncoated fasteners)
For a deck structure, a wood with preservative rating UC4A is recommended. (Ground contact, higher preservative chemical levels.)
Good luck with the rebuild!