NC/LC with parent 1 for many years. Their choice and I was ok with it. And then NC/LC with parent 2 for many years - which meant the relationship with parent 1 improved enormously after that.
In both cases mental health issues and addiction were at play (theirs, not mine). Parent 1 spent the NC/LC years putting themselves back together, doing therapy, acknowledged we had a broken family and did their very best to make ammends. He wasn't perfect but I am eternally grateful to him for trying and loving me enough to try. It was sad that it took Parent 2 to be out of the picture for us to be able to have that level of honest relationship though.
Parent 2 did not, could not, would not accept they had MH and addiction issues. Even to the point they were diagnosed in 90s and hid it from everyone including their long term partner. They chose to gaslight me, turn wider family against me, and contorted themselves inside out to try and prove to everyone I had the issues. I was totally enmeshed so when I finally snapped (pregnant with DC1) and refused to help them cover up their issues, it came as a huge shock to everyone. Myself included. I know for a fact Parent 2 did not ever believe they had done anything to trigger this and their ability to mask the addiction and MH issues was second to none. I was always honest with Parent 2. They just could not cope with it. Indeed I think their MH issues probably meant they didn't remember their poor behaviour at times, which - now I know more, have seen the diagnosis and discussed it with their clinical team I understand - is common. But it was highly convenient for them when wanting to play the victim in all of this. It also meant they could very convincingly tell people they didn't share my experiences and I had made it up, because in their head I had.
Going NC with a parent is never a decision taken lightly. It always brings pain. My hypothesis is there is frequently some deep trauma behind it, either in the individual going NC, the parent who is estranged, or even perhaps in the partner encouraging NC. It is the ultimate response to a toxic drama triangle, where the only option is to remove yourself from the pain - even if sometimes the source of the pain is misplaced.
For all my own experience, I really feel for those of you juggling NC issues due to coercive control of another. I have had friends in similar situations and it nearly broke them. Be "Parent 1" in my scenario for your child... do what you need to, to be there when things change. Don't be Parent 2. It hasn't had a happy ending in my world.