I note that there are no provisions here for secession of nations. Oversight, deliberate, or covered elsewhere by other provisions and principles?

Ah, secession. Let me explain.

No, there is too much. Let me sum up.

Ultimately, the problem here is ontological. Specifically, while we’re used to (and our nation-states are, in many ways, used to) thinking of themselves as lumps of sovereign territory that happen to have people living on them, the Empire thinks of itself (because of its origin as a union of polities that were themselves outgrowths of overlapping PPLs) as a compact between sophonts that happen to live on huge tracts of land.

To an Earth nation, sovereignty (over territory and all that happens therein) is a Westphalian fundamental. To the Empire, and those others that follow its pattern, sovereignty is a bundle of a few specialized property rights which it acquires, often purchasing, over the territory in which its subscribers are domiciled for the sake of ease of administration. It’s a distinctly secondary matter, enough so that there’s a serious if minority Senate branch (the Disillusionist Tendency) that firmly believes getting into the whole sovereign-rights-management business back when the PPLs transitioned into the Old Empires was a terrible misstep that should be corrected forthwith – and doing so would leave the Empire still there.

A secondary complication is that what makes one an Imperial citizen-shareholder is signing the Imperial Charter and purchasing a citizen-share. That’s it. Something that is both (a) entirely disconnected from where you might happen to be domiciled (permanent non-resident citizenships are a thing, even), and which (b) is an individual contract, not a collective contract.

The combination of these things makes secession a hideously complicated problem. For one thing, it’s easy enough for a nation here to decide to secede, what with collective decision-making and territorial sovereignty, but citizenship there is a matter of individual contract with the Empire, which any would-be secessionist nation, even a former constituent nation entire, has no power to break. And I would deem it very unlikely that the Empire is going to kick out citizen-shareholders who want to remain such, even if it could find a way to weasel that through as “due process of law”, which is itself very doubtful.

So the secessionists are going to end up with a whole bunch of permanent residents with Imperial citizenship scattered through their territory, and all the galaxy knows how the Empire tends to react when someone abuses, expropriates, or otherwise mistreats their citizen-shareholders. (That is, after all, one of the things that makes it a premium citizenship brand.)

And then there’s the question of those sovereign rights, which the Empire technically has paid for, one way or another, and thus owns. And, as a several matter from citizenship, has no particular obligation to sell just because the people who own other property rights over the same volume have changed citizenship; people and volume aren’t tied together.

But even in the best case, when the secession is amicable and the secessionists are more than happy to buy those rights back from the Exchequer: Is the Empire going to sell the sovereign rights of its citizen-shareholders out from underneath them to a governance that obviously wants to do things that the Empire doesn’t approve of (whatever it’s seceding over), technically several or no? I do not think so, somehow – and that’s assuming that there isn’t a held-in-trust clause in there that means they can’t sell them to anyone but the holder of the ownership right anyway.

At which point, whether it keeps them or returns them to the holder of the ownership right, the map of this seceded territory is starting to look downright fractal.

(Bear in mind that with the exception of that extremely circumscribed eminent domain clause, it’s not like the Empire has the power to force anyone to move if they don’t want to. If Citizen Lived-In-This-House-300-Years-And-Dammit-I-Ain’t-Movin’ declines to go voluntarily, then he has just created an Imperial Exclave the size of his yard and there’s not a damn thing anyone can do about it. Short of starting a war.)

So, to sum up my summing up, it’s deliberate in the sense that this was a giant can of worms which everyone greatly preferred not trying to solve, especially at the birth of a new empire of ecumenical ambition.

Besides, why would anyone ever want to leave anyway?