macOS High Sierra’s final official version is 10.13.6. The version “10.13.99” does not exist in Apple’s release history. This article addresses users searching for the latest compatible iTunes for High Sierra, likely assuming a theoretical final build, as well as how to obtain iTunes on legacy systems.
No, but syncing may fail or cause data inconsistencies. Always back up your iPhone to iCloud before syncing with outdated iTunes. itunes macos 10.13.99 download