The tailscale clients are, I believe, open source. It’s just the server that’s not, and you can run the unofficial but well supported “headscale” as a server if you want. But this requires you to run this somewhere publicly accessible, like a VPS, for coordination and NAT-punching purposes.

But! I’m pretty sure as the business operates right now, that tailscale doesn’t have access to the actual data connections or anything, it’s all encrypted, they’re basically just there for simplicity and coordination. And their business model is to offer simple things for free, like small numbers of devices, with the hope that you like the service and convince your business to pay for the fancy version for money. So I don’t think it’s quite as bad as the typical “free because I’m harvesting your data” models.

That all having been said, I run headscale 😛