inbox.trash is useless, because array keys are case sensitive. It is
INBOX.something in courier.

If some folder is special for courier, latest versions display XMAGICTRASH
capability. Magic folder is configurable and courier does not document any
commands that allow to detect it.

Courier can store folders in trash even when folder is special for courier.
Allow storing folders in trash. Setting is not enabled by default, but
scripts follow $delete_folder setting and don't add any overrides to courier
setups