As we know there were no corpse effects since AOE3 vanilla. They just simply fade away. But I think it would be a nice addition to the atmosphere of the game.
I don’t think it was because of 3D graphics. AOM had such effects. Maybe it was a performance issue.
You don’t need to have the very detailed “decaying” effects like in old AOE1 and 2 (which was also simplified in DEs). That may require a lot of additional modeling for each model. I think there are many clever ways to make it look good enough without too much work. For example maybe a little more blood and a “sinking into the ground” effect rather than directly disappearing.
This could be an option that one can toggle, or a free DLC.