The index of an ISO file is essentially a table of contents that lists all the files and folders contained within the image. In the case of the MS Visio 2010 ISO, the index would list all the installation files, including:

While it may run on newer systems like Windows 11, there is no guarantee of stability Where to Find the "Index of" ISO Files

Unlike official downloads from Microsoft, an ISO from a random index of directory has no SHA-1 checksum or digital signature you can trust. Once mounted, a poisoned Visio installer could infect your entire network.