hau Ken, very good questions.
First thing to remember is that even though there is no arsenic in the newer treated woods, there is still quite a lot of copper sulfate in even the non-ground contact products.
Now, knowing this, the question of will it harm your mushrooms,
the answer is not really clear cut since you did mention that this is not "new wood"
The short answer would be YES, however this answer would be for those who aren't able or willing to do some research on their own. Or they may be from the "all chemicals are bad and so
should be avoided" crowd.
The best way to tell would be to take soil samples and have them tested for cupric sulfate quantities, this will give you a definitive answer to go forth from.
It is also good to remember that all organic things are made up of chemicals and minerals. These are the building blocks of life.
Issues arise when there is too much of a good thing or there are things that are downright dangerous to living organisms present.
For example growing food in soils that are rich in things like lead, mercury, uranium, etc., might be wise to avoid at all costs.
In my opinion, I would do as John mentions he did and give some extra space to the fence, no need in taking unnecessary risks after all.
The Stropharia rugoso annulata is a good mushroom for mitigation of sulfates, the hyphae will, over time, take in the sulfates and break them down, making the components of these compounds available to other organisms for incorporation or further break down.
You are correct in thinking that vinyl chloride compounds are worse. It is also correct to think that most of the leaching from the current fence is probably over at the ten year old mark.
So, if you are worried, do a soil test and if you aren't really worried, make close and frequent observations and maybe have a few mushrooms tested for "bad" compounds.
This way you will not put yourself in a straight jacket. I would expect a soil test to show some leachate present but probably not in harmful quantities inside the mushrooms.
It is always best to err on the side of caution when talking about things you want to ingest.
Redhawk