They should know only if I ask them to. I don't want to share this sort of information to everybody in my contact list by default. If I think somebody needs to know, I would tell them. If I don't think they need to know, then they shouldn't be told.
Anyway, the consequence of Signal being this way is that I refuse to install Signal at all, and don't recommend it to any of my friends or family. Instead I tell them that everything on phones is insecure and they should never believe or behave otherwise.
If you do not want someone to message you, why have you given them your number?
My assumption would be that if you trust someone enough to give them your number then it shouldn't matter if they know that there is another app that they can use to message you through. Of course this is only my assumption and I am just curious as to your reason why you would not want some particular person to know that they can message you through Signal rather than some other, less secure, app.
To be honest, it doesn't really bother bother me, but just because I communicate with someone does not mean that I want to share any information beyond the contents of my communications. We might both be Signal users, but you're not my friend. If you were, I would already know it.
I guess people could call this a growth hack, but without this kind of thing it's likely the number of signal users would be 10-100x smaller and the network effect would mean it's near useless for most.
Just imagine someone installing signal, not seeing anyone they can chat, and never using it again.