Similarly to MC-256534, I'm getting the impression that experimental items shouldn't be obtainable in non-experimental worlds. However, the following command gives you a shulker box with a bamboo block inside, even if 1.20 features are disabled:

/give @p minecraft:shulker_box{BlockEntityTag:{Items:[{Slot:0b,id:"minecraft:bamboo_block",Count:1b}]}}

It is also still possible to import experimental items "indirectly" using saved hotbars:

1. Open a world with 1.20 features enabled.
2. Grab a shulker box and a bamboo block.
3. Put the bamboo block in the shulker box.
4. Break the shulker box and save it to your saved hotbars.
5. Open a world with 1.20 features disabled.
6. Grab the shulker box from your saved hotbars and place it.
7. See how the bamboo block is inside, free to move around.

I'm guessing instead of shulker boxes one can also use chests with NBT data, armour stands with NBT data and so on.

In multiplayer contexts, malicious players can create experimental items this way, making it potentially impossible to upgrade the world to future versions of the game.