What is the oldest macOS version that will support iOS 15

I don't know that there is a clear answer to this yet. For iOS 14 you need iTunes 12.8.2.3 or better running on OS X 10.11.6 or better. iOS 15 is available to the same set of devices that ran iOS 14 so perhaps OS support will remain the same.

tt2

Hello porkpolitan,

Welcome to Apple Support Communities. We understand that you want to know the specific macOS version that will allow you to sync with iTunes. We are happy to help.

macOS Mojave or earlier can be synced with iTunes. Here’s more information on syncing: Sync your iPhone, iPad, or iPod using your computer - Apple Support

Let us know if you have further questions.

Take care.