My family has one who walked away, and every single day the thought of him not being part of our family again brings me to tears.
It’s my youngest son, he’s 25 now, and whilst we had our share of dysfunctionality over the years (hey, what family doesn’t??) we all remain oblivious to the actual why.
The last time anybody saw or spoke to him (apart from an encounter* I will mention below) was Boxing Day 2010…after a pleasant Christmas enjoyed by all at my daughter’s house, he returned home and that was it. When he hadn’t returned any phone calls within four weeks, it was then disconnected, so I sent him a message on FB. A month later, he changed his name…but my sleuth daughter discovered his new FB name and I’ve been messaging him regularly since to no avail.
We all tried contacting his RL friends, but it seems they had been sworn to secrecy too. Three years later, and my heart still bleeds.
Melodramatic? You betcha. When your own kid disappears, it’s like a death you can never really grieve. You are consumed by guilt (but don’t know why), sadness (but we know he’s alive because otherwise I’m sure his friends would contact us) and anger that he doesn’t care enough to at least explain why he doesn’t want to be part of our family anymore. I’d respect his wishes if he could bring himself to do that.
So this is the other side of the coin. Please, if you are considering cutting off your ‘dysfunctional’ family, let them know you are doing so. Just disappearing is a cunt of a thing to do. Maybe even sending them an email once in a blue moon, just to let them know you’re OK and doing fine. You don’t have to engage them if you feel it’s bad for you, but don’t pretend you’re dead, please. 
*We have a spectacularly unusual surname so my daughter (again) thought to see if he had a LANDLINE somewhere last year. And lo and behold, there was indeed a LL registered in his name in Sydney. So my daughter rang him, and he hung up. So I rang him (from an unfamiliar to him LL), he answered, I said, “Hi John, it’s mum”. "He said, “Sorry??” I said “Hi” again. He hung up again. Even though he only said one word, the relief that washed over me was amazing! But it was short-lived…the sadness, guilt and anger are still a daily experience for me.