The CODESYS Group maintains an FTP/Archive for registered developers. You typically need a or a free registration to access the "Download Archives" section where 2.3.9 is stored as a .exe installer.
Since CODESYS 2.3.9 is legacy software (released around 2006-2008), it is no longer hosted on the official CODESYS website, which now focuses on version 3.5 and 4.x. codesys 2.3.9 download
Opening a project created ten years ago often requires the exact environment it was built in to avoid library conflicts. Where to Download CODESYS 2.3.9 The CODESYS Group maintains an FTP/Archive for registered