That's not true. I left Malaysia, went into Thailand without presenting them any return flight. I left Canada, flew to Thailand, and they did not ask for a return flight.
I just got stamped in from Malaysia last month. They did not ask for a return flight. I was at Chiang Mai airport. But, I had the AirAisa app open with a cheap flight ready, on my phone in case I had to book it. It was smooth as silk. I remember thinking, "That's the easiest immigration stamp I've ever had". 30 days, done.
Im from Seattle and use skyscanner to book out of Vancouver. I do one way trips. I have done a 30 day stamp and two non immigrant O for volunteer jobs. Or get a 60 day tourist and extend for another 30 days for 1900 Bhat.