Do old, dry boards continue to warp?
Regardless of the wood species boards warp for several reasons. Primarily it's when a difference in moisture loss (or gain) occurs at different areas of the lumber. Grain direction also effects movement.
If your boards have been out of the elements for 8 years and have remained straight then they should remain so. I would be more concerned about the boards decaying at this point. Unless they are protected from rain and sunlight they will decay from fungus, termites and U.V damage.
Place them several inches off the ground with 2 x 4's (stickers ?) spaced about every 3 feet. Stack the boards tightly together (if dry) and than cover with an exterior tarp.
8 years is a lot of time for lumber to remain unscathed from the elements. They should be protected or used.