Apple announced in June that Safari would block legacy extensions installed from outside the Extensions Gallery. Safari will now rely on app extensions as a replacement, Apple said that app extensions dont see any browsing details, are more segregated from user data, and put much less of a strain on overall performance. Implementation of this policy has been abrupt, with little explanation for users on why their extensions are being yanked. Some extension makers like Adblock Plus have released new versions to comply with Apples security requirements.”]
Source: https://blog.malwarebytes.com/security-world/2018/09/safari-users-where-did-your-extensions-go/