The dev isn't "legally required" to put in the game anything he had promised.
15 U.S.C. § 45
15 U.S.C. § 1125(a)
If they make a reasonable sounding lie to make a sale, they can be sued, fined, or forced to give refunds. That is for sales, not supporters on Patreon. Based on precedent they would be required to give refunds.
If you didn't buy the game you are not entitled to anything, and since it was stated on the sale problem then it is not their problem in the slightest. If it ends up in the game, optional or not then they have fulfilled their obligations. They have a vision and can carry it out how they see fit, so long as they fulfill their promises to their customers.
