I quite often forget an important upgrade and top player also forgot some. And we discover it very lately that we have forgotten one. For example Hera in his last youtube game “a true test of micro & apm” discover that he have forgotten horse collar when he reach castle age and want to have heavy plow.
It could be wonderfull to have like unit and upgrade queue display on the top left, the same but for undone upgrade. It display upgrades that you haven’t done and are available (for your civ and age) for the selected unit or building. And if you have selected different units or buildings it display nothing.
It will also help new player to know what the have to upgrade, for example they have CA and they can easily know that blood line apply to CA but not cavalry armor.