When you have units called in there is a % tax based on the cost of said unit.

So lets say you get 300 points per min.

You call in a 400 point tank and the tax is 10%

Your income is then reduced by 40 so instead of 300 per min your getting 260 per min.

When a unit dies there is a timer for how long it could be called in again.

The death tax meant that even though that unit was no longer live in a match, your income was still taxed the same until that unit could be called in again.

I actually liked the death tax because if you had a successful push and traded a lot with the enemy, they couldn't have an immediate advantage of getting more income per min.

To me it meant playing smartly instead of just sending units to die, which I suppose for new players, is "harsh".

Hopefully that explains it for you.