Some items like commandblocks or the new wither- and enderdragonspawnegg are only available for operators but I found a way to get these Items without the operator permissions if you are in creative mode (for example if it's an creative mode server). You only have to follow the following steps:
1: Go to a world where you have operator permissions (for example a singelplayer world)

2: Put all the items you want to have in the other world in your hotbar

3: Save the Hotbar with C+1 or C+2 ...

4: Go to the world where you don't have the operator permissions

5: Load your hotbar with X+1, X+2 ... (the number you used to save the hotbar above)

Some items like commandblocks or struktureblocks you can only drop (not even place) but some items like the wither or enderdragonspawnegg you can still use and spawn withers or enderdragons.