For a small LLC game publisher; do prototypes fall under "Purchases Withdrawn for Personal Use"?

Probably not. In order for them to be considered withdrawn for personal use, it would be withdrawn for you to use for your own enjoyment or personal reasons not related to the business.  If the game prototypes are part of your business activities, they would not be considered withdrawn for personal use. 

If you are developing the games, then this would fall under the category of R&D or research and development. So you could list it as Other Miscellaneous Expenses and just type in the description box prototypes or what it is for along with the amount you are spending on the prototypes.  You will find the Other Miscellaneous Expenses Under Less Common Expenses in TurboTax Online or In the Business Expense section in TurboTax Desktop.