Stefanie, I did not want to teach you things you know; excuse me. But mind that there are many readers of our forum who do not know these things. I (and you) must be aware of them when writing things here.
But you are right: I did not reply to your main question: Is that type of colouring at the throat and head of the Pekan Nenasi type of male nagyi specific for that type?
The answer is: no. The other types show these colours too. Maybe there are some faint differences but they are not as striking as one could imagine and as would be a reason for mentioning this soecially with Pekan Nenasi.
But you have another question: Is it not necessary to include this into the species descriptions?
The asnwer is: Partly it is mentioned, if you mean the original desription of nagyi bei Dietrich Schaller 1985. But: That is a short description. For modern standards it is too short. It is not as short as the descriptions of harveyi and allani bei Barbara Brown (which, in a way, are no descriptions at all, only announcements of descriptions that never followed). But the rule is: The first descritions is valid, and so are that of Brown and that of Schaller.
But there is another thing to be said: The description of the colour of a new species must not be “complete” but it needs only to mention the distinguishing markers that make the species a difference to other species formerly described. Take the latest descritions we have got, that of gunamwani and phoenicurus only some weeks ago by Schindler and Linke. Take only phoenicurus: Many of the striking colour features if that species are missing, are not mentioned at all. But that was not necessary. They confined themselves rightly to the distingushing markers.
But perhaps you did not think of the scientific desciptions but of the decriptions that I wrote for this homepage in the “species”-category. Well, I shall have a look on them again. But they have a clear intention: to help peoplce to identify their licorive gouramy, not to describe the colours as fully and comprehensive as possible. That would not add to identifiability. But, as I said, I shall have a new look on them. I shall not exclude that you pointed out a valuable thing.