Do you need to show proof of a return flight when arriving in Thailand on a TR visa?

Last time I arrived I was asked for departure flight information by immigration. It was a first.

I’ve flown in & out of Thailand over 80 times and never been asked for proof of onward flight by immigration but the airline check-in desk asked to see my onward flight details several times.

If you find your airline requires a return flight within 60 days, you can buy a fully refundable ticket to a nearby country and cancel it after arrival into Thailand

Unless you have an extensive history in Thailand, you are not likely asked for proof of onward travel ticket within approved stay, accommodation, and funds of 20,000 baht cash or equivalent in a major currency at the passport control upon your entry.

But the chance to be asked is not zero.

Better to check with your airline about their policy on return tickets. Usually, they don't ask for an onward ticket if you have a valid visa. But there are some airlines that require you to have an outbound ticket when you travel on an even valid tourist visa.