The LH policy is clearly posted on its website. Whether intentionally or not, the LH rule expressly prohibits face shields onboard and makes no mention of whether those are worn alone or in addition to a mask. I suppose that nothing prevents you from bringing one along, but you should be prepared to pack it away or bin it.
"Throughout your stay on board, you must wear a mouth-nose-cover. Face shields and FFP2 masks with a valve are not permitted on board, as they allow the air to escape to the side and do not provide comparable protection.
If you do not wear a mask and do not meet the requirements of this exemption, we must exclude you from transportation."