Block layout - List of all blocks
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