I've had this as my Slack status at work for 3 years now and either people have learned this independently over time, or it has done its job.
I also do keep it as a firm boundary - if people just say "hello" or "how are you" I ignore the message until they actually say why they want to talk to me.
At first I was worried about seeming like a passive-aggressive asshole, but it has really improved my quality of life. No more spikes of anxiety when I get a "hello" and have to sit there and be like "ah fuck what's this bullshit gonna be about" while deciding how and when to answer. Now I can take a quick glance and decide whether to respond now or save it till later.