Open Source USB Flash Drive Circuit Anywhere?
The last of the "easy to find, easy to obtain datasheet" was the SMSC USB97C242.  As memory sizes rapidly increased it was obsoleted a few years after its 2002 introduction.   

Closest thing to that you will find today is a dedicated USB to SD card controller. USB2240 comes to mind.  Use with commonly found SD Cards. You could use a eMMC NAND memory device (which has built in wear leveling) if you wanted to create a custom board and print a custom enclosure.