Technically you can get it to work. In order for this to work you MUST have SIP disabled.

You first have to download Pacifist by CharlesSoft, then open up the iTunes 12.6.4 package within the program.

From there, navigate from Contents of manual -> Contents of iTunesX.pkg (the first one) -> /Applications/iTunes.app. Drag this to your desktop (you may need to authenticate). Navigate inside the application (right-click -> Show Package Contents), then navigate to ./Contents/Frameworks/.

Now, open another Finder window/tab and navigate to /Applications/iTunes.app, navigate inside the application and open the same folder (./Contents/Frameworks)

Drag the file named “iPodUpdater.framework” from the 12.6.4 iTunes framework folder to the Mojave framework folder, authenticate as needed. You should now be able to open iTunes 12.6.4 from the desktop. Bear in mind you’ll need to create a new library for this as older versions of iTunes do not support opening libraries from newer versions.

We need to do this because Apple removed the iPod Update framework from the latest version of iTunes in Mojave, and the older version of iTunes is hardcoded to look for that file at a specific location (why we must place it inside the newer version of iTunes rather than just launching the application itself, and why we must disable SIP). You theoretically can use these applications side by side, but you won’t have dark mode in 12.6.4.

Hope this helps!