Smileless
Your perspective could be coloured by your experience, but I generally haven't seen people go NC that commonly unless there's a major issue. Usually IME it's suggest to start with less drastic measures:
1) See the problem parent only when their biological child is present, so the biological child can handle any situations that come up
2) Call them out on inappropriate remarks. State that if they don't drop this subject the visit is over - and follow through.
3) Limit contact to once a month or maybe less.
4) Change the subject.
NC is generally meant to be a "last resort" for people who simply will not listen to reason. "No, it's not okay to promise our kids things that we don't agree with." "No, it's not okay to undermine our rules." "No, it's not okay to make inappropriate comments." "No, it's not okay to question our parenting."
IME, at least, generally people don't want to go NC. It's often very heartbreaking, especially for the AC. What I have seen is that often the AC has had major "guilt" buttons installed by the parents, and has spent their whole lives living in a toxic environment. Look at it this way: there's a story about a couple who have a next door neighbour. The neighbour likes to dry her clothes on a wash line. The wife is constantly griping about how the neighbour's clothes are always dirty. One day, she mentions that she thinks the neighbour finally learned how to wash her clothes properly because they're clean. The husband says that he actually washed their windows.
Sometimes there genuinely are co-dependent or equally unhealthy situations going on. Because the AC has been brought up in these situations, they're not always able to recognize that this is not normal behaviour. Admitting that a parent is not a safe person is tantamount to admitting that you were never safe during your whole entire childhood - and safety is one of our most basic human needs. Sometimes a third-party who doesn't have all of the history is better able to see what's going on than someone in the midst of it. Often the parents are very upset, because what they've been doing has worked for THEM for so long, that they don't realize (or don't care) that it's actually hurting their AC.
Now, I'm not saying this is true in EVERY situation, but it is certainly a common narrative that I have seen.