Hi,
Welcome to our new Microsoft Q&A Platform.
Yes, Microsoft Application Compatibility Toolkit 5.6 download link has been removed from Microsoft download center. Because this tool is used for Windows 7 and Windows Vista, those versions of Windows haven end of support.
Now we need to download and install the latest Windows ADK, application compatibility tools has been included in it.
Download and install the Windows ADK
https://learn.microsoft.com/en-us/windows-hardware/get-started/adk-install
Regards