So in the case of well meaning laws and judicial systems, there is a protection from harm in the system.
Problem is, that still isn’t a good way of protecting from harm, and more about getting money for harm already done. (as exampled in the article.)
If your ex is abusing you and you don’t know their legal name, then it shouldn’t matter. You know their name - you know to tell people. and if you’re thinking of suing them, I imagine a few steps before that would be informing the people in your community.
I don’t know a perfect solution to stop abuse, or similar things, but not knowing legal names, is not going to make an actual difference.
I personally made a new FB account, haven’t touched the old one, but I was thinking of setting it up as “this person has died” account, with my account as the caretaker