This question is mainly about New Zealand. We call it de facto marriage when a couple lives together as husband and wife, but did not perform the legal paperwork of a marriage.
In many cases, the woman will take the man's surname, and they may describe themself as married, even on legal documents such as electoral rolls and later marriages and death records. Close friends and family may be unaware that they were not legally married.
In this circumstance, children's birth records declare both parents but the child's surname is written on the record as the mother's legal surname.
However, the child is only ever known by the father's surname, including on school records, marriage/death records, and so on.
In this circumstance is it allowable to use the father's surname as LNAB? I think it is somewhat confusing and disrespectful to have someone's profile be identified by a name they never would have used at any point in their life.
The https://www.wikitree.com/wiki/Help:Name_Fields#Last_Name_at_Birth does seem to include "...they never used their birth name" as allowable rationale for not using the name on the birth record, in the context of adoption, but could that also apply in the *de facto* context?