I can confirm that after 34 years of coming to Thailand it was not the immigration officers that asked for the onward travel. It was asked for by the airlines only. The most recent time was last year. Vietjet check in was looking for my visa, I didn't have one when coming back to Thailand. So I explained that I am only going to be in Thailand for 29 days on visa exempt and I showed my flight back to my home country which was on my phone. They entered that into the computer and I boarded my flight without incidence.
Nope, no ID. Tip, I bought from the Aurora gold outlet. They are in a lot of malls and when you sell back to them show the original receipt and they don't charge commission.