Always verify the file signature if possible. These are official Apple CDN links, but good practice: check SHA-1 hash if you have an original reference.
For most modern users, the convenience of managing apps on the phone outweighs the benefits of this older software. However, for digital collectors and archivists, iTunes 12.6.3 remains an essential tool. itunes 12.6.3 download
If you already have a newer iTunes installed: Always verify the file signature if possible
https://secure-appldnld.apple.com/itunes12/091-87822-20170911-6F27FF2A-BF45-11E7-80D2-91CFAAC43605/iTunes12.6.3.dmg for digital collectors and archivists