In the United States, power transformers were still being manufactured with PCB dielectric oil, as late as mid-1979. I do not know if there was action taken on items outside of the utility industry prior to that.
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In the mid 1980's I was an EE with a large petrochem company where we had our own substations. We had to test for and label every transformer and power capacitor that had PCB's. Then they began to require us to do special mitigation around those sites (soil testing, impenetrable berms and firewalls). The goal was to get us to replace or refurbish those devices to eliminate the PCB's.
Sometimes we had to do (repeated) oil changes on big (ten ton sized) transformers and lab testing on the old oils until the PCB levels fell to below regulatory threshold levels. Other times we had to field-flushing with companies that purged out the PCB oil that penetrated everything. A few times the transformers or caps had to be pulled and sent in to have the wood framing inside replaced with stuff that hadn't absorbed PCB's.
Little known fact; Many large transformers in substations have the coils held in place with a complicated array of wooden frames. It is left to soak in the oil and provides the mechanical rigidity to hold the coils and to maintain electrical insulating properties away from the shell.