After upgrading to macOS 10.13.4 my extensions in Safari were disabled. Safari Preferences Extensions tab says "Extensions can be enabled in the Develop Menu". However when I try to uncheck "disable extensions" in the Developer menu nothing happens and it stays checked.
I "fixed" it by deleting the 'Safari Extensions List' Keychain item and then restarting. I believe this is related to some Keychain issues I've been having because the Reset Password screen appeared after I restarted (after 5 mins of a black screen). After I reset my password my Safari Extensions were back but my Keychain had been blown out.
In my case I had to trash all of my keychains, log out and then log in again.
Once the keychains were re-created, I could access my extensions in Safari.
I'd also note I was having difficulties logging into iCloud before trashing the keychains. This issue was also resolved at the same time. Anyone having both iCloud sign-in and Safari extensions issues both, definitely try trashing/moving your keychains.
Warning: if you're not using iCloud Keychains, you'll need some way to recover the contents of the keychains you've trashed/moved.