Won't do a fucking thing. Criminals who assault cops will still be shot, sjw idiots will still screech and riot, and the dumbfounded leftist politicians will stand around going "but wait, we have this 'law'..... WHY ISN'T IT WORKING?!?!"
Except that the cop has to show it was necessary. It lowers the hurdle for the cop to be prosecuted.
Which would result in police being less likely to confront criminals.
If I know that approaching that guy over there, could result in me going to prison because some camera at a bad angle, looks like I shot an unarmed person...... Then I'm not going to approach that guy over there.
I'll wait until he kills someone, and then apprehend them when I know some idiot with a cell phone, won't try and get me sent to prison.