If they're not at the Reunion with no explanation we can safely say they're dead.

I already deemed them dead during the Zap Apple Episode.
Everyone at the School shows up with their Parent and they talk about their Job or some Story.
Applebloom, wants to bring Big Mac or Applejack, but they're too busy with the Harvest. (Why not go to Momma Apple?)
Also that whole thing with the Zap Apples is pretty important to the Apple Family.
So Why aren't Momma and Papa Apple there?
To have them show up out of the blue would just, I don't know, It wouldn't feel right.