DISCONTINUED. Safari 12.0 radically changed its extension system. Apple's goal was to increase Safari security. Unfortunately, not all of Apple's choices in blocked extensions made sense. JS Blocker is IMHO the most unfortunate casualty. It no longer works with Safari and will not be updated to fit into Apple's requirements. JS Blocker was brilliant, incredibly useful and relatively easy to use. I will miss it. If Apple does in the future allow for extensions like JS Blocker, I will be very grateful.
I wrote to Travis Roman, the developer. Here is his reply:
"Apple has dropped support for what they deem "legacy extensions". JSB will never work again going forward unless Apple reverses this decision. Sorry about this, but there's nothing I can do...."
BTW: If Safari give you a popup warning saying "Safari can't install this extension. An error occurred while installing the extension "JS Blocker 5".", what's going on is Safari attempting to inexplicably install the remaining version of JS Blocker in the now legacy Safari extensions Gallery. Apparently, Apple hasn't yet cleaned out its Gallery of blocked extensions. The old Gallery is being deprecated in favor of the a App Store equivalent anyway. To stop this inexplicable popup warning, uninstall JS Blocker from your extensions.