Please dont bite my head off but have you checked all emails from warhorse since 2014 including in your spam folder? Have they had your correct shipping address this whole time? Im sure backers would have got an email in the last six months to confirm all their details so they don’t send the game to the wrong person.
Also when you fund a game through a backing scheme you are essentially donating money to the project and what you get in return is a free gift. I wouldnt expect a refund from a donation.
i just read this on the website under the backers section:
Full Game
Kingdom Come: Deliverance will be released February 13th, 2018. We will distribute the rewards in the first week after the release of Kingdom Come: Deliverance.
To get your copy, please login to your profile on our website and connect it to your Steam account any time after this date. The game will be added to your Steam account and the loading starts. The game will be playable on February 13th, but the exact hour depends on your time zone. PLEASE BE PATIENT
We’ll include the pre-order bonuses to all tiers to show our gratitude to the backers!
NOTE: Platform update, changing the tier or refunds are not possible anymore.
Its still the week following release so I wouldn’t be worried yet. Have you given Warhorse your Xbox account details? you might find that the game is available for download on the store already.