The latest update to Safari (Version 12) brings a range of features intended to improve online security and privacy. Unfortunately one of the consequences is that only Safari Extensions available from the App Store are enabled and you will get a message that Safari no longer supports unsafe extensions and you are directed to the App Store.
Whilst I'm sure that extensions from major developers will migrate to the App Store I suspect that those Extensions provided by scientists may well not make the transition. This is a shame because some are very useful. However you can build the extension yourself to get around the problem.
This tutorial shows how to extract the code from an existing extension and then build it using Extension Builder.