There is no VPN browser extension for Safari. In 2018, Apple removed support for traditional browser extensions and replaced it with ‘Safari App Extensions’ that come baked into app downloads from the Mac App Store. Due to Apple’s strict permission guidelines, few apps offer browser extensions for Safari.
If you want VPN protection, your best option is to use a full VPN app and pin it to your Mac’s menu bar. You can configure your VPN settings to run all the time, during specific timeframes, or only when you click on it to activate it. That’s as close as you can get to the seamless experience of a browser VPN for Safari.