[[:Category:head tracking/singular category]]

I agree with User:-sche. I think consistency with part-of-speech headers and other headword templates (===Noun=== ↔ {{en-noun}} ↔ {{head|en|noun}}) is more important than that with category boilerplate templates (which ideally only bots should add, and humans need not think about).

It also makes sense conceptually to have category templates use the plural while the headword templates use singular: categories contain multiple words, but the headword template deals with only one word. The second positional parameter to {{head}} is not merely a category name fragment; it is also used to distinguish lemmata from non-lemmata now, and even more semantics may be assigned to it, which may make pluralisation even more questionable.

Pluralisation in category names can be handled by a case-by-case mapping. Inventing a new part of speech is something that needs community attention anyway.