Note: In 24w18a the "Update 1.21" experiment is added to normal gameplay, and this bug no longer occurs, ostensibly. However, the bug is not completely fixed, as it would occur again sometime, if another experimental option with a new structure is added to the game.

So I have changed the title of the bug report, and the report shouldn't be closed before the bug gets fully fixed.

You can use a structure block to load a structure of Trial Chambers, even if "Update 1.21" experiment is turned off.
However, in this situation the structure cannot be fully loaded. The blocks that exists in vanilla with experimental options off can be loaded, and blocks that only exists in the experiment are replaced by air. The structure looks incomplete and very strange.

Steps to Reproduce:

1. Create a new world with experimental options off.
2. Place a structure block.
3. Load the structure

   minecraft:trial_chambers/corridor/entrance_1

   .

4. The structure is partly loaded.

Observed Behavior:
The structure is partly loaded and looks incomplete.

Expected Behavior:
The structure should not be loaded at all.
Instead, an error message should be shown, telling players that it's not allowed to load this structure.