We've done a lot of work for 4.0, but many old blocks are still available by default for users to add.
For example, the navigation block. However, given the amount of work we've done on navigation we don't want every teacher being able to add this block. So we could disable it in the admin settings.
This issue is to review each block, whether it should show up in the list or not. And if not, to consider deprecating it.
Edited to add: