Any sex links can be considered a high production breed, also, so add them to your list.
8 is a great age for a chicken! Good job with her. It could be her, does she lay still? Usually hens with reproductive infections will stop laying or start not laying as much as normal. I know that it is normal for an older hen to not lay as much, though.
A few other signs of reproductive issues also include going from laying regular eggs to laying oddly shaped eggs, cracked and mended eggs, spending too much time in the nesting box without laying anything, and eggs with really watery whites.
Any chicken who has an egg break in her or even partially in her, is at risk for a reproductive infection. So really it could be anyone, but they can be so Sneaky about hiding illnesses, so it’s tough to tell sometimes.
Lash eggs are pus that accumulates in the reproductive tract, usually the oviduct, I believe. They’re typically from a bacterial infection in the reproductive tract and aren’t anything contagious. When an egg breaks in or near the hen’s vent (or other part of the reproductive tract), it creates a nice warm breeding ground for bacteria. That bacteria can travel back up the vent and spread infection, causing the pus accumulation. The pus can be all over in the abdomen and sometimes causes swelling of the abdomen, even. Lash eggs are pus from the oviduct.
Here is an informative article about lash eggs:
https://the-chicken-chick.com/salpingitis-lash-eggs-in-backyard/
You shouldn’t have to worry about your other chickens catching anything from the sick hen, but ensure they have enough calcium available and clean nesting boxes to lay in.