Thinking About Rudy's Death in Undertale
So, as we know from the Cancelled Christmas Watch Dialogue (Which is a real thing that happened, I have to remind myself), Rudy Holiday's Undertale counterpart is fallen down and likely deceased by the time the game takes place, as Asgore "never got to show [him] the sun".
However, we also know that his death must have been fairly recent, because we now know from the 10th Anniversary stream that Noelle's Undertale counterpart was a small child at the same time.
So if Rudy died that recently...
Why isn't he part of an Amalgamate now?
Just about every other monster NPC with a deceased relative got that relative back in the Post-Pacifist section of the game, but if Rudy was among them, then Asgore definitely would have mentioned that; he's fully aware of Alphys's experiments now, according to Politics Bear.
This, as far as I can tell, can only mean that Rudy's fallen down body wasn't donated to Dr. Alphys. Asgore only asked, after all; it wasn't a government mandate or anything.
ASGORE asked everyone outside the city for monsters that had "fallen down."
So the person with say over Rudy's remains - most likely Carol's Undertale counterpart - refused to donate his body to the Royal Scientist's experiments, and Rudy passed away naturally.
The Amalgamates are obviously public knowledge. How long do you think it took for Carol to put two and two together?
How long do you think it took her daughters?
Carol's probably kicking herself for that one, but if she had known, would she have even done something different?