Agate
I can guess two possible options for the kandra.
1. God Sazed endowed the gift of presence on the now mistwraiths.
2. Some of the kandra survived in the cave with the Terrisman and people of the city, along with the small mistwraiths, these are re-born with the spikes they pulled out during the resolution.
I can imagine too that some kandra on assignment may have hidden in the shelters with the rest of humanity.
Brandon Sanderson
The kandra.
Yes, they live. The people were smart enough, eventually, to replace their spikes. (And there were a couple who were on assignment who made it to storage caches.)
However, there will likely never be any more of them, since Hemalurgy is required to make them. They are now some of the few people who can communicate directly with Sazed, who—like Ruin—can whisper to people most easily when they are connected to him via spikes. With some speculation, you can probably guess what kind of roles the kandra will end up playing in future books.
Kaimipono
On a broader level, is Hemalurgy officially dead, then? Or is it still extant in some Ruin-free (but still messy) form? (If it's gone, is there any imbalance since Preservation's magic power is kept and Ruin's isn't?)
Brandon Sanderson
Is Hemalurgy dead? No, not at all. It, like the other two powers, was not created by Ruin or Preservation, but by the natural state of the world and its interaction with the gods who created it. It still requires the same method of creation, but very few people are aware of how it works.