I am in the middle of demo and rebuild of 40 feet of softit removal and raising all of my ceiling up to 8ft.
The house was built in 79.
I have been re working the existing wiring so I can reinstall all the dry wall.
I found two strange usages of 14/3.
The home builder ran 14/3 for two different circuts instead of running 2 14/2.
So, instead of running 80 feet across the house with 2 14/2 he ran one 14/3 and shared the N.
In the living room the red hot wire goes thur a light switch to control a plug.
The black controls the other 4 outlets, garage lighting, door opener, utility rm lights, kitchen, hallway and entry way lights.
(Today I ran 2 new 14/2 feeds removing garage's 5 items, and another removing the utility and kitchen.)

Another situation they did this was in the kitchen, (same idea but with 12 awg wire) Red wire feeds the disposal, black the dishwasher and the counter outlets.
Does this type of shared N still happen today?

Also, I just bought a GFI outlet as my kitchen does not have one. Can you use a GFI with a circuit that has a shared N ?
Or will turing on the disposal trigger a GfI on the other circuit?

Thanks
Bill