Please don't ever suggest burning old fence panels or wooden sheds.
The old way of preserving thin section timber was tanalising.
Planks of timber were loaded into a thick steel walled pressure chamber and a vacuum applied.
The solvent-based tanalising liquid was then pumped in, with (hopefully) all the microscopic airholes in the timber filling with the tanalising fluid.
The pressure chamber was then returned to normal air pressure and the timber unloaded, allowing the solvent to evaporate.
Quite apart from the solvent contributing to NMVOCs, (non-methane volatile organic compunds), adding to urban smog,
the active material in the tanalising liquid was copper chrome arsenate.
Any of those three constituents will kill you,
you and your neighbours don't need toxic smoke in the local atmosphere,
especially as we currently have the cleanest air in living memory.
Any old fence panels and sheds can be safely deposited at your local Councils domestic recycling facilities, when they re-open.
tl;dr: don't burn old fence panels or sheds.