"I just wish people wouldn't tap on the shoulder when I have headphones on. It's a sign of "I'm working on something, go away"."

True, we have a convention that you - unless it's really important - don't tap someone on the shoulder if he/she is 'plugged in', but mention him in Slack. That way one can seriously stay focused.

"Btw. NC-headphones are not complete solution as they can pass nearby human speech through quite clearly."

My Bose QC20's actually filter out speech really well, only high frequencies can be heared a bit, but not enough to be distracted by imho :-)