From Safari 13, you can only install extension from Mac App Store.

brew install [email protected]
export PATH="/usr/local/opt/[email protected]/bin:$PATH"
# make sure node and npm is v10, cause octotree used gulp 3, which is not working with node 12.

git clone ~/src/octotree
cd ~/src/octotree
git checkout master

npm i
npm install [email protected]
npm run dist
# extension locate in ~/src/octotree/tmp/safari/octotree.safariextension/

cd ~/Library/Safari/Extensions
mv ~/src/octotree/tmp/safari/octotree.safariextension .
  1. Enable Developer menu in Safari
  2. Developer - Show Extension Builder
  3. Add octotree.safariextension and Run