As the days grow longer, you start to realize... wow, this deck is getting kind of old and rickety. Maybe I should do some outdoors renovations before the heat of summer comes on. And then...
Dear Don't Move Firewood,
Can you burn lumber? What about left over treated boards from building our deck?
Burning untreated, unpainted ends of new clean dimensional lumber (i.e. the last few inches that you trimmed off your pine 2"x6"s) is totally fine for home use, or backyard firepit use. So go ahead and burn it on site, on that same property, if you would like.
However! Please take note that many campgrounds ban burning dimensional lumber scraps not because they are harmful in general, but because they are not easily distinguishable from construction scraps that might contain nails, staples, etc that could injure workers or visitors. Thus, even though it would be harmless from the sense of pests and pathogens to bring clean new lumber scraps for burning while camping, we do not advise doing that because you might have them confiscated anyways.
But the other part of your statement worries me. It is never a good idea to burn "left over treated boards." The chemicals used in the treatment of boards are, by definition, toxins. Burning toxin laded wood is a bad idea whether you are burning it inside or outside. Most treated wood contains chromium, copper, and arsenic. Chromium and arsenic are both extremely bad for you to inhale, especially while burning.
All treated wood scraps should be disposed of in a solid waste facility, i.e. a landfill. Don't burn them!