I think it would be safe for Microsoft to release (intentionally or unintentionally) the source for just about any product. I bet it's incredibly difficult to run a successful build. From talking to someone who used to work on Excel, it took them around 1 day to build Excel from source. And that's if everything goes perfectly and you know exactly what you're doing and are using the build system and setup and configuration that the Excel team has in place.

People have built working operating systems from both the XP/Server 2003 and NT 4 leaks. Here's someone building Server 2003 on Windows 11: https://www.youtube.com/watch?v=AWZe00v2Rs0