I have several pieces of clothes with yellow old stains.
I use a bleach pen with Spray and Wash.
Fabric content? I would try an oxiclean type treatment first.
You can make your own Oxiclean which will get rid of yellow stains: https://www.craftylittlegnome.com/homemade-oxiclean-recipe/
Oxiclean works great!
Have you tried a DIY oxiclean? It's great for yellowing.
Soak in Oxiclean overnight.
I bet this works with clothing too: https://www.hometalk.com/36610742/cleaning-yellowing-pillows-and-more