macOS upgrades have been free since OS X Mavericks[0] (10.9, released in 2013). macOS 10.13 (High Sierra) came out in 2017, so is a 5 year old OS at this point[1]. It also supports all Macs that macOS 10.12 (Sierra) supports[1]. And you don't need an Apple ID for anything other than Apple services and the Mac App Store. OS Upgrades don't need an Apple ID (even on older versions of macOS where they're installed through the Mac App Store), and you can even network boot a recovery image and install the latest version over the network[2] (Intel Macs only).