But you are voluntarily giving that info away if you use WA and don't change your privacy settings. The 'last online' feature is quite important IMO, and if you're concerned about this then disable it.
Edit: On the other hand it would perhaps be better to have it on 'Contacts only' as the default, but you could still monitor your colleagues as they probably have you in their contacts anyway.
Whatsapp has no setting to disable the "online" flag when i have opened the app, and a bunch of things mentioned can be built on top of just this.
I think this shows how some seemingly trivial data points on an individual level can allow one to build something way more than the sum of parts at a mass surveillance level.
I agree but I don't believe the average user realizes that it is possible to do that with the above data. And indeed, to do that manually is not practical, but if the info becomes scriptable, it's another matter.
Scriptable and available for anybody. That my contacts in WhatsApp can correlate my activities with my status, this is part of the purpose of WhatsApp, that any stranger having my mobile phone from my business contact can do it, this is another matter.
Edit: Discovered that one can share only with "My Contacts" these information. I am surprised I haven't seen this before or maybe the if you do not share, you cannot see info box misguided me into not restricting to my contacts these informations.
Edit: On the other hand it would perhaps be better to have it on 'Contacts only' as the default, but you could still monitor your colleagues as they probably have you in their contacts anyway.