My best friend of 30 years, (Rachel) has a cousin (D) who was raised by her grandparents because her birth-mum (G) had her at 19, and didn't want her. G didn't tell anyone she was pregnant til she was 6 months in, and it was too late to do anything about it then.
She gave birth to 'D' and her parents took D on, as G didn't want her. G was very abusive and aggressive towards her daughter (D) from when she was 3 or 4 years old, (possibly started before but she could only remember that far back.)
She bullied her and goaded her, (along with her aggressive boyfriends who also thought it funny and clever to taunt and bully a small child) and made D a really quiet and timid child. D told her grandmother, and a couple of aunts but was told to 'grow up,' 'stop being silly,' and 'stop telling lies.'
D thought G was her older sister until she was 9 or 10 when people started telling her she was her mum. D went NC with her birth mum (G) when she was about 16-17 years old, and left the house when she came to visit. She completely ghosted her. Because of the aggressive and abusive behaviour she had endured over the years. G didn't care much, and the fractured and toxic relationship just died.
Around 13 years ago, (2009 - when D was 25 or so,) her grandparents died, only 4 months apart, and the wider family (D's 4 cousins and 2 aunts and 2 uncles,) sided with G, said she was young when she had D, and she should have cut her some slack, and not ghosted her.
D explained that G had been very abusive and aggressive, along with her boyfriends, even pretending they had snapped her dog's neck, and that her grandad (who she thought was her dad at the time) was dead, and also telling her (when she was 5) that no-one loved her and she would be going into a childrens home soon. One of the boyfriend's exposed himself to D when she was 7 too.
In addition to all this, 'G' regularly slapped D, punched her, broke her things, and just made her life miserable when she was around her. No-one believed any of that either. They didn't believe her when she was a child, and then they didn't believe her when she was in her mid 20s, and trying to explain WHY she had ghosted her birth mum.
When she explained (in her mid 20s when her grandparents died,) WHY she had ghosted G - her birth mum (and she wasn't believed, AGAIN,) the wider family started to distance themselves from her, and she wasn't invited to things, (family weddings, and Christenings etc,) and was constantly left out of everything.
Eventually around 5 or 6 years later in 2015, my BFF Rachel (who had stayed in contact but kept her at arms length, so as not to fall out with her mother who had ghosted 'D') texted her to let her know her uncle Pete had died. (Although it had been 7-8 months since they had been in touch last.)
The text came back undelivered, and they never saw D again. This was about 6 or 7 years ago. My friend Rachel went to visit her house a few weeks later but she and her DH and 2 kids had left, and no-one had a forwarding address. My friend looked for her on social media but couldn't find her (and her name is unusual too - very unusual, like say 'Desdemona Jefferson.' I mean a really unusual name.)
Then she looked up her 2 DDs, Emily and Olivia (not their real names) and bingo, she saw some of their photos 'liked' by a person called 'Dezzie J.' And a pic of the cousin popped up. Her location - some 50 miles north in another county.
My friend messaged her and said 'hey Dezzie it's me Rachel, your cousin, how are you?' When she went back to see if there's a response, 'D' had blocked her. So had the 2 daughters.
A few weeks later, Hannah, one of the younger members of the family who remembers D and her 2 DD's, sent D and her 2 DDs a message too, and she was asking why D had lost touch with everyone.
Clearly, the younger family members knew nothing about what had happened, and were badgering her to get in touch. Again, Hannah was blocked. When my friend and Hannah tried to get in touch again, both D's AND her 2 DD's facebook accounts had been deactivated.
This was 6-7 years ago. No-one has seen hide nor hair of 'D' since. They looked several times on social media, but neither her or her 2 DD's are coming up anywhere.
As has been said, there are often very good reasons why people 'disappear' and don't want to get in touch again. Being rejected by your birth mum, having no-one believe she and her boyfriends abused you, and then having the vast majority of the wider family, avoid you and ghost you for 4 or 5 years, after your grandparents/adoptive parents died, will MAKE someone want to move away and have nothing to do with their whole family.