Even if you’re still running macOS High Sierrra (I’m on 10.13.6) you’ll receive a Safari update (from 12 to 13). This will break a number of extensions – including most adblocking – also a vim-based keyboard extension I was using called Vimmy.
So you may want to avoid upgrading, or do as I’ve done and switch your default browser to Firefox etc. for the time being.