Modern Windows does not "activate" VS6 over the internet. The installer only checks if the key matches the checksum algorithm. Therefore, any valid checksum key works.
Many repacked ISOs remove components, add unauthorized tools, or break the installer. You might end up with an unstable development environment that introduces subtle bugs into your legacy applications. Microsoft Visual Studio 6.0 ISO With Key
: Some versions accepted sequential patterns like 0123456789 or 1234567890 . Installation Tips for Modern Windows Modern Windows does not "activate" VS6 over the internet