Since the snapshot 23w04a it's no longer possible to shift click items to the second anvil slot.

Steps to Reproduce:

1. Join a world.
2. Place an anvil.
3. Place any suitable item in the first anvil slot (e.g. a diamond sword)
4. Try to shift click any suitable item into the second slot

Observed Results:

The item you wanted to shift click to the second anvil slot did not move to the second anvil slot.

Expected Results:

The item you shift clicked should move to the second anvil slot