In any case there cannot be a good reason to share the online status / last seen date before any interaction has happened between two contacts, and approved by the receiver.
If you have had no interaction with someone, you should first accept a message (and not report it as spam) before this information is shared. Ideally that would be the default as well for the About field and the profile picture.
I definitely agree – none of my hypotheses make for a good reason. I'm just wondering if there's anything technical behind what seems to be a quite stubborn decision that also sticks out (as everything else has controllable privacy options).
Another weird decision is that read receipts are not possible to be deactivated in group chats, but there is no explanation for that that I could think of (delivery receipts might be required for faster encryption key ratcheting, but the read status has no significance at all for the protocol).
If you have had no interaction with someone, you should first accept a message (and not report it as spam) before this information is shared. Ideally that would be the default as well for the About field and the profile picture.