Lubing the backing plate only lasts for a while, maybe 6 months to a year. You can grind the right angle end of shoe material to a radius and it will stop it forever. Did it to both my Focus after getting tired of relubing the backing plates. This is for squeal in a slow roll type traffic jam, when brake is barely on. It used to drive me crazy. Hasn't done it in years now and shoe material or brand doesn't matter.
With light brake pressure the right angle corner digs in and then snaps loose at high frequency, the backing plate is really a bit too thin and rings easily like a bell. Lubing rub points kills the vibration transfer to the backing plate to ring, but wears off with added brake dust. Knock the shoe end corner off and shoe cannot dig in anymore to vibrate. Don't even need to do all 4, just the bottom of back shoe and top of the front.