How to disable service workers in Safari 12.1? (Mac)
You can try to turn off the feature via the Debug menu.  For me, using Safari 12.1.1, It is listed under

Debug -> WebKit Internal Features -> Service Workers

Note that I am referring to the internal Debug menu, and not the Developer menu.  The Debug menu is activated by writing the key IncludeInternalDebugMenu  into Safari preferences (e.g. defaults write com.apple.Safari IncludeInternalDebugMenu 1 -- under Mojave, you will have to grant Full Disk Access to Terminal.app first; otherwise the setting will not take.)

I'm not sure if this persists across browser restarts, though.