What does the units icon above their heads

If you're talking about the ranking icons then no. Dev confirmed it is just for looks. No actual impact to the game.

bit of a superfluous nonsense tbh, it used to have meaning when units could morph into other units but its only value now is to tell you which units are doing a good job

if someone decided to remove it, I wouldnt complain

and I love complaining

actually could you post a screenshot so we can see what icon you mean? i either never saw what you two saw or managed to ignore it 2 and half a year long...

I thought you made them not vague, and the progression was 1, 2, 3, 4, 7, 10 times cost damaged.

If it's only superfluous then get rid of the icon just for cleanliness. It would be cool to see how many kills a unit has in the selection window at the bottom though, like SC2:

Kill indicator would be more useful if it distinguished between (say) Flea and Cyclops, and if your units would occasionally turn on each other for 'ks'.

Should I add making a loading screen explaining this bit to my todo list?

Why not instead reward those units with a tiny buff (nothing new here)? E. g.: Maybe an "elite" Fencer could get a very slight increase in range, or something like that, and another level could, for instance, make it set up a tiny bit quicker. It wouldn't be enough to throw off balance, but it would encourage a little macroing, especially on newbie level. Would that not make these icons more useful?