There are no VPN browser extensions for Safari. However, using a VPN’s app will keep your entire Mac protected, including everything you do in Safari. On top of that, some VPNs (like ExpressVPN) can be controlled from the top drop-down menu bar on your Mac — it’s as convenient as a Safari extension and keeps your whole device protected (instead of only your online activity within the Safari browser). Apple stopped supporting many third-party Safari add-ons with the release of Safari 12 (in 2018) and started charging a fee to anyone who wanted to keep theirs in its gallery. That’s the reason why you won’t find a VPN extension for Safari.