1] Notepad Opening an XFDL file in Notepad is a no-brainer. Right-click on the XFDL file, hover to “Open with” in the context menu, and select Notepad. It will open the XFDL file in the Notepad.

Updates to Notepad and other apps could roll out before then, though, in keeping with Microsoft's "release them when they're ready" approach to feature additions in the Windows 11 era.