This release covers the same revisions of WebKit from Safari Technology Preview 57, but includes new Safari and WebKit features that will be present in Safari 12. The following Safari 12 features are new to Safari Technology Preview 58:
Icons in Tabs. You can enable showing website icons in tabs in Safari’s Tabs preferences.
This is great. I don’t want to run Mojave betas, but I’ll gladly use Safari Technology Preview builds.