How do fix Safari extensions on Safari 12?
The latest Safari update (12.0, Sept 17, 2018) turns off some and and stops supporting other extensions. 

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/).

Update: Some unsupported extensions work again in Safari 12. E.g.,


The "Zotero Connector" extension for Safari (https://www.zotero.org/download/) now works for me even though—as I write this—it does not show up in Safari's list of supported extensions in the Mac App Store (https://itunes.apple.com/us/story/id1377753262).