As we all know when you set up a new mech you adjust the limit screws so that the chain doesn’t pop off the granny ring when changing down or off the crank side when changing up. So why has my chain started coming off the granny ring after some time of working perfectly? No-one has altered the limit screws. Any theories on this and what I can do to stop it? I know cables stretch but surely thats why you have a limit screw.

Maybe the whole mech has slipped/been knocked down slightly on the seat tube?

Cables strecthed, springs stretched and just generally worn.