When you have installed Safari 12, you notice that Silverlight no longer appears in Safari preferences in addition. The plugin does not work anymore and you can not reinstall it.
We are waiting for an update from Microsoft so that the plugin works again under Safari 12.
Until that time you can use Firefox 52.9.0. Silverlight has already been optimized for these browsers.
We recommend you to turn off automatic updating in the FireFox settings.