I've seen some chatter about Safari 13 that says it breaks legacy Safari plugins and requires all plugins to be downloaded from the App store.
One thing I've also noticed is that it seems to have broken OS X Dashboard Safari widgets, too. Now when I select "Open in Dashboard" in Safari, my content just says "Missing Plugin". I haven't found a workaround for this yet and I know that the Dashboard is going away in Catalina, so it's likely that Apple make not care too much to fix broken Dashboard widgets.
If anyone finds a workaround for this, please share.