The latest Safari update (12.0, Sept 17, 2018) turns off some and and stops supporting other extensions.
E.g., many people use Pocket's Safari Extension to save items to a OS-independent list. But I don't see Pocket on Apple's new list of supported Safari Extensions (https://itunes.apple.com/us/story/id1377753262). That might be why Pocket now recommends that Mac users just use their Mac app: https://help.getpocket.com/article/1042-using-pocket-for-safari
Pocket may update their extension such that Apple starts supporting it again. Until then, there might be a javascript bookmark(let) for saving items to Pocket (https://support.apple.com/en-us/HT4550) or a Pocket email address to which you can send items to your account (https://getpocket.com/add/).
But are there fixes/workarounds that would allow no-longer-supported Safari Extensions to work in Safari 12?
