> It would be more privacy preserving to prevent that user from ever knowing the answer until I send them a message.
Someone has to send the first message. To have end to end encryption, that someone needs to be able to retrieve the other person's public key in order to be able to send it. This means they can determine whether you have a public key, i.e. whether you have the app.
> The usual areas this is a problem are for example: bad exes, poor former business relations, anyone who is harassing/stalking someone, and so on.
If your bad exes don't have your phone number, they don't learn that you installed Signal. If they do have your phone number, they learn that you installed Signal, but if them learning this one bit of information is an actual problem for you, maybe you should change your phone number.
Someone has to send the first message. To have end to end encryption, that someone needs to be able to retrieve the other person's public key in order to be able to send it. This means they can determine whether you have a public key, i.e. whether you have the app.
> The usual areas this is a problem are for example: bad exes, poor former business relations, anyone who is harassing/stalking someone, and so on.
If your bad exes don't have your phone number, they don't learn that you installed Signal. If they do have your phone number, they learn that you installed Signal, but if them learning this one bit of information is an actual problem for you, maybe you should change your phone number.