It's rather funny to me that I still get notifications that someone I knew ages ago (but are still in my contact list) are now on Signal.

I find it funny rather than scary because I knew the tradeoffs Signal decided on even before I started using it. The fact that you use Signal itself is not a secret. If you need that to be secret, use a burner phone or something else. But I'm pretty sure not everyone else knew that when they joined.

In one case, I got notification that a guy I knew in high school had joined Signal, he was pretty far left then, and googling him I found out he was extremely far left now (splinter of a splinter of an anti-electoral Maoist group). I sent him a friendly note welcoming him, and explaining the basic thing I've explained here, that everyone can see you joining signal.

I never got a reply.