Mojang was username+password later email+password, following industry trends (username is easily seen in-game so you only have to guess a password which for many people is easier than guessing their email).

It continued to be so, and then Microsoft forced everyone to switch to a Microsoft account, deleting the account of anyone who didn't do so (in complete violation of Notch's original sales contract with players). If you watch the progress bar of Prism Launcher logging you in to a Microsoft account you see it logs into Microsoft, goes through a bunch of indirection, logs into Xbox (!!), bunch of indirection, then logs into Mojang. And then you get to launch the game.

It used to be a simple endpoint with username+password -> auth token. Not any more with Microsoft!