In fairness, they do out of the box support most of the major players in the SSO space for free (google, Microsoft, okta, gitub...). I'm happily using Google SSO to manage a small number of users for my tailscale account free of charge.

FWIW, tailscale is technically not free once more than 1 user, its a commercial service at the end of the day not some self-hosted open source application. They have extremely generous fair use policy in my experience and won't bill for a small number of users though. This means custom SSO is likely only a real issue for paying customers to begin with, although I can understand the frustration for the few users who do want it on the free tier.

> https://tailscale.com/pricing/

Take a look at Headscale if you want to self host. https://github.com/juanfont/headscale