I haven't used this hook before, so I may misunderstand something.  I'm on the new christmas update that just released this morning.

I put this code in my plugin (from docs):

object CanDeployItem(BasePlayer player, Deployer deployer, uint entityId)
{
    Puts("CanDeployItem works!");
    return null;
}​

1. I confirmed my plugin is loaded (other hooks are working).
2. I placed a foundation, then a tool cupboard on top of it, then a bear trap nearby.

Nothing was written to console.  Do I misunderstand what "deploy" means?

If this turns out to be a bug, is this still the right place to ask about it, or is there a separate forum/bug tracker for official support of the API?

If it isn't being called for how you are using it, then it isn't intended for those items.

Is there a list of items it's supposed to work with I can refer to?  Is there a different hook I should be using for tool cupboard placement?

You can look in the patched server DLLs from Oxide to see where it is hooked and get a better idea. I don't have it in front of me, so not sure at the moment.

Looks like locks and a few others.  I managed a super awkward workaround for TCs using the entity spawned event.