Suspect old-school nappies were soaked more due to old-school detergents!
That and lousy washing machines too I believe!

Solutions I have come up with are to hang the stained one in the window or if it's just the boosters then a soak in napisan on occasion can brighten them up. I'd not risk an AIO with PUL or brightly coloured minky too often though. I have read that you can make a paste from napisan and a bit of water to just put on the stained area but I've not seen any ratios of how much or tried it myself to see how it works.
Are you removing the poos straight away? I know it sounds obvious but I only get staining if I am out and the nappy is dirty in a wetbag for a few hours or I'm out and my DH changes the nappy and he throws it in the laundry for me to deal with when I get home.