Citizenship is a modern institution; yeah, the Athenians had it, if you're OK with a notion of citizenship that extended only to a small percentage of the population. But even in ancient Athens, the majority of the population were slaves.
In feudal Europe, most people were serfs or vassals; they came with the land, a bit like trees and game. The only people with rights were aristocrats, and then only really if they had land. Poor people might have had some rights in law, but the judge was the local baron; it was meritorious for the sovereign to promulgate "the King's justice", but it didn't happen much.
The change came with the Age of Revolutions; rights are something you have to seize. To my fury, I remain not a citizen, but a subject, because the English Revolution was led by landed gentry, not by the populace.
The idea of universal rights is a fine idea; but not having been seized, they don't exist.
It pretty much is. A government does govern two things, a population and a corresponding land area. You may try to have one without the other, but that would lead to massive problems because of the way humans and their home area interact.
If you govern a population without a land, you are practically instantly at war with some other population because both populations will want their way of life as well as their property rights enforced around the place they call "home". For a current example, see the palestinian exile population, who have a government but no land of their own, thus leading to constant conflict with their host countries.
If you govern a land without a population, you are lacking any kind of compass and attachment to values. Land alone is a dead thing, and a government cannot just be recruited from land, it has to be people doing the governing. Basically, there is nothing to govern without a society.
Citizenship can have a number of definitions, but the loosest one is something like "currently inhabiting the land area of that corresponding government". You may change those definitions, introduce various classes of citizenships, modify the ways in which it can be obtained. But for the aforementioned reasons, any definition that doesn't involve something like "a citizen is strongly associated with a land area and comes from a corresponding populace (governed by a corresponding government)" is a weak and fragile definition that will not last the test of time and human interactions. Note that the government part is in parentheses, because actually governments are far more interchangeable than population and land area.
"Its two headquarters buildings in Rome enjoy extraterritoriality".
The article claims they have no territory, but what they mean is there is no territory capable of supporting a military + institution. I don't know this is a practical counterexample.
Nevertheless, it is interesting to note that clubs and private militaries, and independent autonomous intelligence agencies exist, even if they are on the extreme end of what could be considered a "government".
How so? A government must have those it governs, those must be then members of the set of people whom it governs.
You can call it something besides "citizenship", but maybe we're talking about different terms, or you have a more technical definition implying more than simply membership under a governing party?
Either way, this is the basic definition of a citizen, so trying to divorce the two seems futile to me:
"A citizen is a member of a political community who enjoys the rights and assumes the duties of membership."
They govern over non-citizens as well, e.g. over residents and visitors to the geography where the government has/claims sovereignty, but the non-citizens tend to get fewer rights than the citizens.
I understand your distinction I think...however the parent comment (about abolishing citizenship) is still (on the face of it) advocating anarchy. Degrees of citizenship must exist always, even when the "official" line is that none do - as you state even in the case of "equal citizenship" there is still the degree of citizen, and non-citizen, so really you can never remove that (without also removing government).
Being in the position of abolishing _birthright_ citizenship is a very different stance than abolishing the practice of citizenship outright. I think you are speaking of "second class citizenship" e.g. immigrants or temporary who do in fact benefit from and are expected to follow the laws of the land, but who do not get e.g. voting rights.
I.e. perhaps the view is the equalize all practical citizens under the law (which is possible but difficult to do wrt to land ownership and community security, especially).
My interpretation of the parent comment was that citizenship should not be restricted by the circumstances of birth.
Countries would still be governed by those who live within their borders, but you wouldn't be discriminated against based on your place of birth, e.g., you could move around freely.
Or, maybe more practically, greatly ease the requirements of immigrating and becoming a citizen of countries.