What is a self-hosted app/os/service/ect that is widely "known" as being fully open source, but it actually relies significantly on closed source software?

I was looking into Tailscale which I thought to be complexly open source, but it turns out that their coordination server is closed source. If you want to run your own open source coordination server, Headscale is the go-to option.

This is no fault of their own (as they freely express this in their FAQ) it's just that I had always been told by people that Tailscale was fully open source. This got me wondering what else is not as open source as people widely accept it to be?