You are correct, Unknown should be used and only Unknown.
Per the help page on Name Fields (https://www.wikitree.com/wiki/Help:Name_Fields#Last_Name_at_Birth) :
"If the last name is unknown, use Unknown in the field. Do not use Not Sure, NN, Wife of X, Adopted, etc."
That said, I would not recommend changing the LNAB from "something creative" to Unknown unless you're pretty sure that there is no way of finding what the actual name was. Why? Because it's more effective to merge from "NotCertain" to the real name, for instance "Brown", than to rename from NotCertain to Unknown and then from Unknown to Brown. See the Multipe Redirects Problem.
On the other hand, if you're creating a new profile and could not find a maiden name after a reasonable attempt to discover it, then use Unknown, nothing else.
There have been numerous discussions about the variants of use by members for Unknown in all fields. Unknowns project has the following List of Unknowns
The Help Page for Name fields has a section about this Special Rules for Required Fields, but people don't follow it.
The 'footnote' on the page has the following
"Using the same convention for unknown last names is important. In the long run, this clarifies things for everyone. It groups these profiles together in various alphabetical sorts. And we can program our systems to ignore profiles named Unknown in certain contexts."
Linda, the provision that the footnote relates to is also important, listed under "Special Rules for Required Fields":
If the first name or last name is unknown, use Unknown in the field. Do not use Not Sure, NN, Wife of X, Adopted, etc.
Edit: Corrected wording. Thanks, Linda.