Wouldn't the same thing work the other way though to the point that calling shields pretty rare in any Medieval period is a bit of a stretch? I just have a hard time imagining the poorer soldiers taking the field without a shield. I know plate armour became fairly common eventually, evident in the late Hundred Years War and War of the Roses, but surely it didn't become that common that there were no foot soldiers who saw a shield as useful anymore.
Surely not. But it is not so easy to tell exactly to what extent. The sources are very scarce when it comes to the lower classes, numbers, role in battle and equipment, in most Medieval battles we hear only of the nobility and men-at-arms. Even in the case of peasant armies we get only the number of knights fighting/slaughtering them. So besides the archaeological record some of our best sources for equipment use is different decrees about what a militia or levy is required to muster with. In many 12th and 13th century sources shield (and kettlehat) is a minimum requirement at least in Norway and Denmark, though peasants were less well-armed in other parts of Europe. And there's plenty of support in the sources that shields were standard equipment until the 1300s.
But from the mid-1400s as you say, armor technology was at a level that you might aswell have your two hands free to wield a weapon powerful enough to wreak havoc on the opponents armor, and generally in the 15th century professional soldiers who as far as we know (again lack of sources) did most of the fighting were heavily armored; knights, men-at-arms, even non-retainers like militia, refrained from using shields (though pavises were still popular among crossbowmen and archers). But certainly a lightly armored man would bring a shield in any case, if he could wield it, logic dictates.
Sources for 15th century shield use includes illustrations in the Codex Wallerstein and the Gladiatoria Fechtbuch, and a description in the Chronique du Religieux de Saint-Denys. All of these portray staged combat, though, and the shields are used to protect against throwing lances then discarded before entering melee.
