Is signal still demonstrating their commitment to my privacy by notifying everyone with my number in their phone contacts who is also on signal when I join ?
That’s a good question I’d like to know, too. Perhaps they hash the phone numbers then compare to their master list for matches?
This form of number is also the one your (mobile) phone actually uses, although they let you type in any sloppy human attempt at a phone number and translate.
Signal apps periodically reach out to Signal's servers to do two things: Confirm that this specific user does still have Signal (and so messages to them should be accepted) and optionally upload a set of these hashes for their contacts to see if any of those have Signal and so messages to those numbers can go securely via Signal instead.