From what I have read. Windowblinds 11 is on the schedule of programs that will soon be under development, but Curtains has trickier issues to fix and is more of a hopeful that has not been decided one way or the other as of now.

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.

I'll take either one ,  Windowblinds or Curtains if they get updated to Windows 11.  I just really love the transparency effects.  That's all I really want.     

I know it's not recommended, but I've been running WindowBlinds 10 with Windows 11 since not long after Windows 11 released. Start 11 and some other software (I don't remember if it's okay to name it, so I wont). There are problems drawing certain Win32 dialogs and menubars, among other things, but I can live with it.