This is a short thread so I'm gonna necro it for context instead of creating a new one. Also, if someone else runs into this while trying to play a dead civ, this post might clear things up.
47.xx seems to have fixed the behavior that allows civs that died during worldgen to stay dead after a player creates a settlement in their name.
First time playing 47.05, I run slabciv as usual and keep generating new pocket worlds until I get one with a slabbed dwarf civ. After that, I embark wherever I please and set up a fort fit to sustain some ~20 dwarves (accounting for the two hardcoded migrant waves). After that it's usually a matter of making them marry, then raising absolute population cap so they have a few children, then waiting for the children to grow, then making the second generation marry, and so on, until I have some 80 or 90 inbred dorfs to do silly shenanigans with.
However, this time around one of my dwarves claimed the title of king "after a friendly discussion with some rivals" as soon as the second year started. Not only that, but a third migration wave arrived.
Just to test things out, without using slabciv I created a pocket world with tons of megabeasts and let it run until all settlements were destroyed, then embarked. First week someone claimed the title of king, as usual from a civ that didn't truly die and still had abstracted population stuck in a limbo.
So it seems that every start of year, the game now performs some sort of check that can revive a civilization if it was previously dead.