The index for the cursor slot used to be 499, according to the wiki, but attempting to detect items in that slot no longer works. The wiki also states that the slot used to be exclusive to survival mode, but it doesn't work for any game mode I tested.

Steps to Reproduce:

1. Set up a command block with the command: 

   execute if data entity <username> Inventory[{Slot:499b}] run say item detected 

1. Add redstone with sufficient delay so that you can open your inventory and pick up an item
2. Set game mode to survival
3. Activate redstone, open inventory, and pick up an item using the cursor

Observed Results:

no chat message

Expected Results:

chat message "item detected" appears