I live in Khon Kaen . Part time for close to the past 20 years, full time in the past almost four.
Never carried my passport , or a copy of it with me. No one (other that at IO, bank etc) has ever asked me for it. I know it is irresponsible , but I often go out having forgotten my wallet. I mostly have my telephone with me, In it I have a picture of my Thai licence and pages of my passport, but I have thousands of pictures, so if asked, I ma not sure if I would be able to find it. If needed I would call my wife and have her text me a picture, but I never had. Not sure about BKK , and I am not advocating behaviour such as mine.