Issue partially fixed in 5.0.6 beta - the installer has full GPLv3, the license.txt contains the full GPLv3, but WinSCP.exe (in the About/License window) does not contain the full GPLv3.
Thanks. I have reopened the bug.
Should WinSCP.exe read the license from license.txt anyway?
No. WinSCP.exe is selfcontained apllication, it does not rely on any other file.