Thanks for the responses here. The story of what was going on here is coming together, and making sense. When I first encountered this, I saw that Category: Dutch Immigrants to New Netherland had not been edited for over 2 years, was marked for deletion, and appeared to be ready to deleted. I thought it odd that there was no recent edit history documenting the request for deletion, and it did not occur to me that it might have been marked for deletion over 2 years ago.
Around the time that category was marked for deletion, some of its sister categories were renamed to a new name format preferred by Categorization (example: English Immigrants to New Netherland was renamed to Category: New Netherland, Immigrants from England) and were removed from the New Netherland Immigration category and placed in Category: New Netherland, Immigrants, which had Category: New Netherland as a parent, but was otherwise unconnected to the New Netherland Settlers Project. Some New Netherland Immigration categories were unchanged.
It seems that Dutch Immigrants to New Netherland was the only category in the group that was marked for deletion. There was no indication of what was wrong with the name, only that it was incorrectly named. Apparently a Netherlands Project volunteer recently discovered the category and recategorized the contents the same way that Dutch people who immigrated to the United States of America in (say) the 1800s would be categorized.
From all of this, I hope I can infer that it is still OK to categorize New Netherland immigrants as having gone to New Netherland, rather than anachronistically treating them as having immigrated to geopolitical units created much later, such as the United States of America.
I don't often look at these New Netherland Immigration categories. I know they are of greater interest to some other New Netherland project members than they are to me. I do check these categories sometimes -- for example, when I am looking at someone identified with an out-of-the-ordinary place of origin and I am trying to remember who else in New Netherland [supposedly] came from that place. I hope the New Netherland project can continue to use categorization to sort the national origins of New Netherland settlers.