Common and Rare troops now drop from the Event Keys.
However, I have the following suggestion to change it.
If you already fully ascended - all the available troops upgraded to the Mythic status and if you have at least four or more copies of each such troops, you should no longer be getting Common and Rare troops from the Event Chests or Keys.
I am suggesting a mechanic that’s pretty much identical to the Guardians from the Guild Chests.
Once you fully ascend all of them to the Mythic status and have four copies of each, they no longer drop.