The update period is usually three years for Safari + general security updates and four years for iTunes (minus one year as High Sierra was released last fall). Third-party browsers such as Chrome are generally supported for twice as long as Safari.
So there's still some time to go before High Sierra is no longer viable, but missing out on features like dark mode on older Macs is seriously disappointing nonetheless. Dark mode should have been added years ago in my opinion.
Agreed. Dark Mode should be an absolute requirement for every app and every OS. I'm pretty annoyed with the fact that Developers of apps and Operating Systems tend to ignore this extremely common request from their users. If I have a choice in apps, and one has dark mode, I choose that app. Bright screens are very obnoxious.