I think we are making ourselves a serious problem. You are in effect making a huge chunk of the radio spectrum unusable. Mains cabling was never designed to be used like this and huge powers are used due to the losses in the system and interference caused by other devices plugged in. If you want a cable then run it round outside the building then?
Most of the interference from the plug adaptors is from about 4mhz up to about 32mhz, they have filters fro the ham radio bands but not for the international short wave bands. The eternal ticking and screeches even about
200 yds away is ridiculous. The other problem is the switch mode power supply. In the old days they were analogue with large heavy transformers but did not create interference. More usually these days there is a very crude rectifier directly on the mains running a high frequency oscillator of dubious purity and a tiny transformer and the other side is then controlled and used to drive your gear. It is far more efficient and if designed without saturating the ferrite transformer and with proper suppression components need not be an issue but it is because they makers pair down the design to such an extent it is just a jammer. Worst ones are often laptop chargers and some led lamps from China. Then there is ADSL once again forcing very high data rates along unscreened twin wire connections originally designed to only carry voice frequencies. These produce an alternating whining noise and a hash over the whole lf bands. When they are underground its not so bad of course. Brian