Troops are found in chests based on their initial base rarity, regardless of how much they have been ascended. Ascending troops does not affect their chances of being drawn from a chest.
This means that Common troops that have been ascended can still only been found in Gold Chests.
As an example, this is a few chests being opened. The Epic troops are still shown as Epics (in purple) and the Ultra-Rare troops are still Ultra-Rares (in blue).
However, select one, and it is clear that this is a fully-ascended-to-Mythic troop.