Can I extend my 60-day visa exemption in Thailand if I book a return flight for 90 days?

@Carl *******

yep, any flight leaving Thailand is all you need.

Technically immigration requires you to have an onwards ticket. They just very rarely check, and when they do it's normally because they're trying to deny you entry. For example, you spend almost a year in Thailand and they feel you're not a genuine tourist and they want to deny entry, so they find some technical reason to reject you. For 99.99% of genuine tourists they have no reason to want to deny entry

Airlines are more likely to check and it seems to depend on the airline and their policy. From Australia I never get checked these days. But from America, Canada there's many reports of people being asked. So it seems to be airline dependent.