The issue about treated timber is a complex matter that few actually study.
Over the years various methods of treatment have been used and some are shocking, but not all.
If you burn any treated timber and eat the
ash you may be ill.
In 2021, treatments are varied to suit the final use of the timber, thus different levels of the treatments [ toxins] are used.
Most house frames are treated timber so the level of toxins is lower, in those than the level used for timber going into the ground..
Dangerous Fumes do not come out of treated timber, otherwise the whole world would need to demolish all new timber framed homes.
Accordingly I would study the timber you are offered and see if you can use it.
Its my guess you can if its treated for use as above ground external use timber.
One of my sources of information
timber treatment facts