That's good to know! My last date of entry was after a one-day trip down to Seattle. On re-entry my passport wasn't stamped. Since we drove, there is no airline ticket etc.Thats true too!
But I don't think so you need to bring any boarding passes, as long as you remember the dates, it should be fine! They already have this info in the system, they just cross check and see if it matches with their database.
Thats what happened in my case!
Before 2017, I used my US passport to travel from Germany (since I needed it anyway to visit the US, i.e. usually Seattle) and it wasn't stamped a single time in 5 years of travels.