Without wanting to gratify the GN perception that we're talking about them (and as I said below I had never even been on GN when I started this thread, so it definitely wasn't about them), I am intrigued by the fact that the discussion here was described as 'nasty' and 'unkind'.
Because as far as I can see, the discussion here has mainly been about people trying to work out how one of the most important relationships in their life became so catastrophically unmanageable that they didn't see any option other than to cut off contact.
But the perception that this discussion is somehow an attack does fit with Issendai's analysis (which various people on here have referred to) that the estranged parents cannot bear to hear anything negative, to the extent that they are genuinely unable to process the actual content of what is being said, and simply hear it as abuse and shouting, even if what is being said is objectively a calm and reasonable request for a change of boundaries. I think that analysis goes a long way towards explaining why estranged adult children have found it impossible either to continue with the status quo, or to change the dynamics to create a more manageable relationship.
From my own relationship, things that I/we have done that have been perceived as an unwanted act of aggression and that have triggered a whole series of hostile letters and emails include, for eg:
Objecting to the fact that my EP had taken items from my home without telling me (she tried to justify this by saying she thought the items belonged to her - they didn't)
Asking her not to keep talking about weight loss in front of my slightly chubby 13yo
Asking her not to make comparisons between the dc's academic achievements in front of them
Declining her request for us to lend her a large amount of money to bail her out of a financial mess entirely of her own making
Turning down a last-minute invitation to lunch because we had pre-existing plans to take the children to an event they'd been looking forward to
The list could be extended indefinitely. The point I'm making is that these are all reasonable reactions and boundary requests, which were made calmly and non-confrontationally, and yet they were clearly perceived, and reacted to, as they were unprovoked attacks and deliberate acts of cruelty. Which I think takes me right back to my OP on this thread, in which I quoted the mother who thought she was 'entitled' to demand a 'list of grievances' from her estranged dd. When in fact it's clear that the reason the estranged dc are unwilling to go any further down the path of providing a such a list is because they know from experience how utterly futile it is to try and communicate to these parents anything they don't want to hear, because not only will it not be heard, but it will escalate the conflict further.
It's a hideous stalemate, because I think most estranged adult children are on some level constantly casting around for something that we could do slightly differently that might provide the key to a resolution. But I fear that when confronted with someone who is genuinely unable to take in anything that isn't what they want to hear, there probably is no way forward. Hence the NC.