Even only with public group messages, you can figure out some others sleep schedule or when is he/she mostly active if you have enough data. This api will only make things worse because it trace you 'actively' now.
Besides that, you are able to send message when other is not here is the biggest point of a text chat app. If you must chat instantly, why not just call with phone?
Not sure I understand your special case here.
If I go out for a walk or I go watch some TV or otherwise stop using my computer, depending on the chat app I want to either automatically be 'unavailable' (for work chat app), or I want to stay 'available' but have notifications go to my phone instead of the chat tab (for friends' chat apps).
That suggests to me that this feature is more than just "did you touch that particular tab", and I'm not really sure how you're reading it differently?
There is always a less convenient and more manual way. Pointing that out helps no one.