Desktop Curtain 3.1.2 is out with a couple of fixes.
In both the App Store and direct versions of the app, the isolate frontmost window feature should now work properly on macOS Big Sur. And for direct users, Spaces support has returned—it appears we accidentally disabled it a while back, but just heard about the problem yesterday. Sorry!
We are not allowed to include Spaces support in the App Store version; if you want Spaces support, please follow the instructions in this blog post to Crossgrade to the direct version.
App Store users should see the update in the Mac App Store app soon, if not now; direct users can update in-app or by downloading a new copy from the Desktop Curtain web page.