I knew a guy who was type 1 at work. The injection was so fast, I rarely saw it, and it was sometimes 10 or 15 minutes before lunch, so didn't necessarily need to be in the actual cafeteria or restaurant. Never saw him ask about sugar content, but assume he pretty much knew what insulin to take for an average lunch. I saw him inject, but I was a friend. My guess is he would avoid doing it in front of complete strangers if he could, but would do it if it was unavoidable.
If I saw someone in a restaurant taking insulin, it would be no big deal. Medical necessity. I've seen seniors take their pills at restaurants - unremarkable.