[2023] Is this a joke or is it actually true? from the FAQ

Originally posted by Daigo: No. Windows 11 is not supported

Originally posted by LoboFH: The game works in Windows 11.

Good point! We've removed that from the FAQ. Icarus runs fine on Windows 11.

Those FAQ were written for the launch of Icarus back in Dec 2021. At the time Windows 11 had only been out for 2 months and some people had questions about how good it was for gaming or not. It's better now.