Safari 12.1 ships with iOS 12.2 and macOS 10.14.4. It’s also available for macOS 10.13.6 and 10.12.6.
The only way to enable it at the moment would be:
Home Screen => Settings => Safari => Enable the Motion & Orientation access setting.
After you enable the Motion & Orientation setting, the Gyroscope feature and also WebVR plugin will work normally.
One good thing is that other iOS browsers – e.g. Firefox or Chrome are unaffected by that Safari change. Those browsers are still allowing gyroscope/acceleration sensor usage by default.
We hope in the near future, Apple would enable the Motion & Orientation setting by default or at least let the developer detect whether that setting is on or off to notify the user.