Walking up to say "hi" and have a casual chat with someone who's deep in focus work at the office is just as unreasonable. You don't know whether people are "available" on Slack because they're open to a casual chat (as at the water cooler) or because they want to be not totally unavailable should something urgent come up (like being at their desk). Since you don't know, it behooves you to treat it as the latter.
