I generally find it unhelpful. People’s reaction time sucks and and so many people hear ‘left’ and jump in that direction before their brain can process. It’s not an intelligence thing that some people try to make it seem, it’s just that the average person has this interaction like once per year thus don’t have muscle memory baked in.

I yell out ‘behind you’ from several feet back or ring a bell if I’m on a bike. Or more often just slow down as I normally don’t really care.