iOS 12 & Yosemite
This tip has been created as a spinoff ofiTunes/Mac OS/iOS compatibility
Its most current version is on my own page*.
iTunes 12.5.5 is available for Mac OS X 10.10.5 to sync with iOS 12.1.4 or earlier.
iTunes 12.8.1 while designated for 10.10.5 compatibility has not been found compatible with iOS 12.1.5 or later
unless you upgrade to Mac OS X 10.11.6. A detail of what is possible is described below.
iTunes 12.8.2 and Mac OS X 10.11.6 is required for iOS 13 and phones released/updated after September 19, 2019.
Two factor authentication logins for AppleID unlocks was introduced on Mac OS 10.12.4. If your iOS is current for it, you can usually use it to unlock either from another current iOS device, or http://iforgot.apple.com if you don't have a Mac.
While in Mac OS X 10.10.5, you can:
1. Use Image Capture to download movies and pictures made with your iOS 12.1.5 or later device.
2. Use iCloud backed up bookmarks, addresses, and calendars and share with your iOS 12.1.5 or later device.
3. Use iTunes purchased movies and music that appear on your iOS 12 device’s Music and iTunes Apps.
4. Create CD ripped files and put on an external storage device to be read one song at a time with your iOS 12.1.5 or later device, whether it is a lightning port data storage device, or iCloud, or other cloud software sharing service that works both on Mac OS X and iOS.
You can’t without upgrading to Mac OS X 10.11.6 or later:
1. Backup stored icon folders of on screen bookmarks from your iOS device.
2. Sync ripped CD content from iTunes.
To upgrade to 10.11.6 or later, you can create a partition that has the newer operating system installed while keeping 10.10.5 for other application compatibility on any Mac compatible with 10.10.5.
Upgrading to 10.11.6 on pre-July 20, 2011 Macs may require installing 10.6.8 first to install 10.11 if you haven’t installed 10.11 before, or you can install Mac OS 10.12 from the App Store if you have a newer Mac that supports it.Upgrading to Sierra
If you have downloaded El Capitan before, it will appear in the purchased section of the App Store.
Note if your Mac does not support Sierra, and you are missing your 10.6 installer disc, Apple still has their retail 10.6.3 available at its online store (not the App Store), which can then be updated 10.6.8 if your Mac is older than March 15, 2010. If it is newer, you may need the original installer discs. However, based on my research those Macs that are newer also support 10.12.
Exception was found to the rule on May 28, 2019: iTunes 12.5.5 (included link is from my website*) on mac os 10.10.5 syncs with iOS 12.1.4 on iphone.
This was discovered quite by accident visiting a customer’s house that had this configuration. So iOS 12 is possible to sync with 10.10.5.
Note: https://support.apple.com/kb/dl1977?locale=en_US refers people to iTunes 12.8.1 and 12.8.2.
10.10.5 Combo & Security Update 2017-003 Yosemite will ensure you can upgrade Mac OS X 10.10 to the latest version of iTunes on 10.10.
*Links to my page may give me compensation.