The bug

When restoring toolbars, items that are normally unobtainable through the creative inventory without the use of commands can be obtained. This can be used by players with creative mode but without operator status on servers to obtain items like dragon eggs, command blocks, structure blocks, and more. 

Steps to reproduce

1. In a world in which you have operator privileges, save a toolbar with dragon eggs, structure blocks, and command blocks, as well as some dirt and other items that are obtainable through the creative inventory
2. Join a server on which you have creative mode (operator privileges are not required)
3. Restore the toolbar, even without operator privileges

Behaviour

You will see that when you follow these reproduction steps, every item is restored, including the dragon eggs, structure blocks, and command blocks. The expected behaviour is that only the items obtainable without the use of commands would be restored.