The same list shows up when you click on Place block. Doesn't look quite the same, but it is.
It was moved because they did UX testing and not a single person managed to place a block in D8, they all completely ignored the right sidebar. So a different solution had to be found, which is currently definitely not very nice yet.
This is the issue: #2513580: Implement the new block layout design to emphasize the primary interaction of placing a block
