Are/is "headquarters" always used as plural?

Nouns that look like plurals, because they end in -s, but whose meaning is collective or composite, are known as ‘pluralia tantum’. Headquarters is one such, and premises, surroundings and outskirts are others. Headquarters is unusual in that it can be followed either by a singular or by a plural verb.

Quarters alone can mean almost any place of residence, including the place where troops are lodged. Ngrams shows quite a wide disparity in favour of quarters are over quarters is and a cursory look at the citations in the OED suggests that quarters in this sense is treated as plural. The OED’s earliest citation for headquarters is dated 1660: ‘The head-quarters of the Army were at Windsor.’ This suggests that the longer word took on the grammatical number of the shorter. The earliest citation for headquarters is is from 1937. The change may have come about through the perception that headquarters is a recognisably single place in a way that premises, surroundings and outskirts are not.

Headquarters is not always plural, nor always singular, as noted in your link. Among responses at that link, "Loob" in England writes:

... if I were talking about a military HQ like the one I used to work in, I would definitely use "is".
But for a company headquarters (= the HQ of a single company), I'm pretty sure I use both "is" and "are". I was putting this down to that free-and-easy way we Brits have with collective nouns ...

Likewise, I imagined that headquarters is would be found more often in British usage than American. However, that notion is not borne out by American- and British- corpus ngrams for headquarters are,headquarters is :

As can be seen, headquarters are is more predominant than headquarters is, in both corpora, and is more markedly so in the British English corpus than in the American English corpus. (Note, there are numerous confounding cases that ngrams does not sort out, e.g. headquarters appearing at the end of a sentence or phrase, and is or are starting another; or reference to multiple headquarters establishments. However, I think those are probably a small enough fraction of instances that the general idea holds.)