Yes. Sierra is still safe. I receive security updates regularly. Even for iTunes and Safari. But I would be using Mojave if I had a Mac with a Retina display. Explaining… something was modified from macOS High Sierra that made text blurry than usual on non-retina displays. System-wide. That's why I still use macOS Sierra. I tried a few times to use Mojave, but I can't tolerate the blurry texts.