So glad to know I wasted my money on this product
Why did you waste your money?
Stardock Curtains works perfectly on the OS it was designed for (and the only one it has ever said it supported). There is no requirement to upgrade to Windows 11 and Windows 10 continues to be supported by Microsoft for years.
What I will say is any Windows 11 compatible version of Curtains wouldn't be free to Curtains 1.x customers as it would be a considerable amount of redesign and thus we would have to recoup the costs.