my sister has pine untreated deck that is 30 years old. its in great shape,stained every 2 years.
i have 3 yr old PT DECK STEPS that are cracked and warped.

I WOULD NOT USE PT WOOD ON MY DECK FLOOR OR STEPS OR RAILINGS.anything that is PT that lies flat and rain/sun hit it will crack bigtime .

wood 30 years ago untreated as john h said,was OLD tree wood.it just did not twist/crack.

today wood DOES.
UNLESS YOU LET IT DRY FOR YEAR IN GARAGE IN SHADE ,IT WILL TWIST/CRACK.you got to flip it and hope you can keep it straight.
PT is just terrible to work with.

i did a 16ft deck with 2x8 as joists.
i knew it would lift and twist.
so, i waited 2 weeks before putting down my IPE WOOD.
i had to take hand planer to LEVEL boards as they lifted and twisted.some twisted badly and i had lots of posts in ground too.

i had to plane EVERY 2x8 to get floor level.