The Tailscale client (non-GUI) is open source: https://github.com/tailscale/tailscale

And they collaborate with Headscale to provide an open-source coordination server (with, unsurprisingly, a more limited featureset, but it works fine with their closed-source GUI client): https://tailscale.com/opensource#encouraging-headscale

I use the combination myself and it works quite well, but of course is less convenient than using their product (which I also do in a different context). Overall I'm pretty happy with their open-source stance.