I was able to straighten a slightly warped 10-foot piece of pressure-treated wood with clamps.
Will it warp again? If so, is there anything I can do so that it doesn’t warp?
This 2x4 is being used in a fence and it has two vertical 4x4 pieces of wood on each side.
As the temperature and humidity change, it will tend to warp again. But, if it's securely fastened to the other fence components, that will help to keep it straight. If it was only a tiny bit warped, you're probably just fine using it. But if the warp was a really bad one, it could do interesting things to the fence as it reverts to its old shape.
Yes. Pressure treated wood will warp unless it is fastened throughout to a set piece. And by warp I include twisting, bending or whatever. The only thing you can do to really mitigate it is a good coat(s) of paint as soon as you install. Rain hitting it (or the rain air) will move it quick.
PT should not be used for fencing unless your design allows you to liberally fasten it. Also it should be thick.