As the title says, this is a Slacker Backer pre-order, not a regular game pre-order. You can pledge via PayPal for copies of the game and selected Kickstarter rewards, as fans requested this payment option during the initial Kickstarter.
The Slacker Backer site opened in September 2015, but only offered the PS4 version. Fans requested the PC version be added to the Slacker Backer site, and they are making good on that promise.
http://gematsu.com/2016/12/shenmue-iii-pc-pre-orders-begin-december-15#Krf2JhQBjTWySEE7.99
Via Famitsu through Gematsu.
You can currently pre-order/back the game on PS4 via the Slacker Backer site.
EDIT:
Regarding the "full-scale production" comment, take a look at their workflow chart released last year:
While this is likely a shortened and simplified version of what will actually happen in development, it is effectively the final hurdle before debugging begins after they have spent a year working on their prototype.
EDIT 2:
Further clarification on what "final stage of development" means from Ryan Payton, who is working on the game in an advisory/production capacity. It is the final hurdle before debug, but this does not mean the game is imminent or near-complete.
The Slacker Backer site opened in September 2015, but only offered the PS4 version. Fans requested the PC version be added to the Slacker Backer site, and they are making good on that promise.
http://gematsu.com/2016/12/shenmue-iii-pc-pre-orders-begin-december-15#Krf2JhQBjTWySEE7.99
Via Famitsu through Gematsu.
Development is entering the final stage headed towards full-scale production. Early in the new year, I think well be able to show you things such as new videos.
We will continue progressing with development so that our backers and those who experience the world of Shenmue for the first time can do so with pleasure.
Also, at noon on December 15, we will start pre-order sales for the PC version of Shenmue III, which received a lot of requests.
Those interested in Shenmue III, please check the official website.
You can currently pre-order/back the game on PS4 via the Slacker Backer site.
EDIT:
Regarding the "full-scale production" comment, take a look at their workflow chart released last year:
While this is likely a shortened and simplified version of what will actually happen in development, it is effectively the final hurdle before debugging begins after they have spent a year working on their prototype.
EDIT 2:
Further clarification on what "final stage of development" means from Ryan Payton, who is working on the game in an advisory/production capacity. It is the final hurdle before debug, but this does not mean the game is imminent or near-complete.