At the end of the third book in the Mistborn series, there is no definitive statement about the fate of the koloss. The author's own commentary/annotation notes:
Oh, and as bonus aside, let me point something out to you. Human and his group of koloss were inside the Homeland when the sun came out, destroying everything on the surface. They were still there when Sazed rearranged the world and fixed things. TenSoon and the kandra were also inside, though they had been turned into mistwraiths. Hum... Wonder what happened to them...
One hint from The Alloy of Law is that Harmony may have saved them and given the koloss their humanity back. One character is even noted as having koloss blood.
Is there anything more definitive regarding their fate than the inference from The Alloy of Law?