As per the 23w45a changelog, it states the following about trial keys:

An item that can only be obtained from Trial Spawners

However, trial keys are also included in the loot tables for trial chamber entrance chests and corridor pots, making it possible to find in those containers.

Steps to Reproduce:

In game:

• Open a world with experiments enabled
• Place down a repeating command block
• Enter loot spawn ~ ~1 ~ loot minecraft:chests/trial_chambers/entrance or loot spawn ~ ~1 ~ loot minecraft:pots/trial_chambers/corridor into the command box
• Power the command block (or set to Always Active), and observe loot drops
  • Observed: Trial key can drop as an item
  • Expected: Trial key does not drop as an item

Files:

• Navigate to the version jar
  • On windows, you can open the run window (Win + R) and enter %appdata%\.minecraft\versions\<version> into the text box
    • replace <version> with the affected version in question
• Open the jar with an archive software such as WinRAR (or make a copy of the jar and add .zip after the file extension and open it like a regular zip file)
• Navigate to data\minecraft\datapacks\update_1_21\loot_tables
• Open chests\trial_chambers\entrance.json and pots\trial_chambers\corridor.json and review entries
  • Observed: Trial key is a loot entry
  • Expected: Trial key is not a loot entry