You're still welcome to ping people on Slack during lunch and see how they like that. More often than not though, I think you'll find that they're going to respond to you once they're back on the clock. It's fine if you're a friendly person, but I think you underestimate how much social pressure is involved in talking to people during their lunch break. The only reason I really engage in conversation while eating lunch is because the alternative of "I'd rather just enjoy my food and save the work talk for work time" is a faux pas.