Yes most airlines travelling into BKK from the SEA region are requiring onward tickets at check-in. You can get that easily online and then not use it for a nominal sum.
My landlord did mine and I have never been asked for it because the IO can see it in the system. If anyone is in doubt they can check into a cheap hostel for one night and they will put you in the system because it’s the law. After that you are good to go without printing. Or just go with screen shot if it eases your mind.
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