I've seen this mentioned before. I'm not sure many players would like this. Some players just like to occasionally play without having to worry about constantly donate or chat in cc. Rather not have other players complaining about you being on and not donating.
It has been ruled out because people need their privacy: having this implemented in the game removes it. An online indicator opens people up to abuse: for example, if they are online but not donating, people could use that as an excuse to abuse that person. Furthermore, there would be strain on the server, as it would need to check who's online and who isn't. Ultimately, this would start making the game lag and potentially turning down plenty of players.
Go visit our STICKY to see any other ruled out ideas.
And then go visit STICKY 2.0 to see why they're ruled out!
Also read Dahimi's guide to posting ideas.