macOS High Sierra 10.13.6 may be one of the final updates to the macOS High Sierra operating system, as Apple in June introduced macOS Mojave, an update that will be launching to the public this fall.
Given that updates to OS X / macOS have not been particularly compelling the last several years, and also given that High Sierra is finally in relatively decent shape... it's likely all the Macs I am responsible for will be riding along with High Sierra for the next three years.
I'd kept stuff on El Capitan until recently - but that likely won't be getting any more security updates once Mohave is released.