[jmprspret]

The clients are the open source part of Tailscale. They can be forked and built by someone else if required.

However I do not think Tailscale is going to remove the custom control URL feature from their mobile clients. For one, I think there are legitimate "Tailscale Enterprise" use-cases for the custom login server.

Additionally, I have heard that Tailscale has been supportive of the Headscale project, providing assistance to the devs.

Further, Tailscale seems fairly committed to keeping their clients open sourced, and engaging in the developer community. Of course as you can say this can change at any time.