Hi Matthias,
I struggled with the same problem - our user base settled with the following: I wrote a plugin that hides the online status on public pages. It removes that info rather than hiding it via CSS, but that solution is global, and no per-user setting. With that plugin, when you have an account and log in, you can get the information anyway. See this topic:
…if one day we get this one (this is what @Mittineague was referring to):
https://meta.discourse.org/t/presence-features-for-forums/12
…then I guess the urge to hide your presence will get more prominent, too…