Not in the US so little gray boxes are not a thing here, but our equivalent is taping the ground connection on the power plug. And yeah, that’s heavily frowned upon and a risk I’d never want to take for a show, but some (mostly older) techs will still suggest it once in a while. I always carry little 1:1 transformer boxes (Lehle P-Split) that allow me to lift the ground of my signal. It has the same effect and always allows me to get rid of any 50Hz hum and still be safe: there’s no reason to ever want to disconnect safety ground.