So dead gods don't really die, they just float, asleep, in the Astral Plane if I remember correctly. Is this the same for 2nd or 3rd edition demigods (1-5 divine rank)?
"A true warrior needs no sword" - Thors (Vinland Saga)
I think this is dependent on which world in D&D you are on when consulting this. I think the "higher" end of the spectrum with more followers has a chance of going to the astral, but I think the lower end of the spectrum is either absorbed by the prime material plane or the plane of their preference.
This isn't verified in any books but it is how I would treat it otherwise.
"These people are my family, not just friends, and if you want to get to them you gotta go through ME."
I'd say it would depend on the astral domain the demigod posseses and the amount of wheight you give in your campaign to achieving ascension.
If you go for the 4ed Exarch explenation that these gods spend most of their time on the Material plane they likely have no domain of their own yet. You can also choose to decide that reaching divinity means you automatically start forming an afterlife on your personal domain in the astral plane. Larger sized home realms provide a diety with more ways to survive a lethal attack (by absorption into the domain or changing its form during a conflict).
Most dead gods whose domain have been destroyed end up in the astral as a giant statue of themselves (its regulated by Anubis, who is the guardian diety of dead gods and the astral planes). I can't remember if the size of the statue is linked to the divine ranks a dead god had. If so, than lesser demigods would likely make only medium sized statues, which are easily lost in the vastness of the Astral planes.