Every single book I ever checked out from KU is on my kindle unlimited page.
That's the one you get to from the your account drop down and just go almost all the way down where it says "your kindle unlimited". There you can sort by all, returned or current. It even kept my record when I cancelled KU for a while and re-subscribed. But I did not see that list when I was not subscribed. I don't think I could as far as I remember. But when I re-joined, they were all there.
My history goes all the way back to the beginning of KU, July 2014,
