Are you talking about visa on arrival or visa exempt entry? What's your nationality?

One would assume that knowing how quickly things can change at these times, airlines and immigration would not insist on a flight out of Thailand.

In general it's not an issue. On my recent flight from Phnom Penh to Bangkok I was asked at check-in for a flight out of Thailand, and not having one was not an issue at check-in, but the staff told me immigration might ask for a flight out of Thailand (they didn't).

I was once asked at check-in (Taipei to Bangkok) for a flight out of Thailand within 30 days. I had a flight out within 60 days and was going to stay visa-exempt with extension, but their rules said 30 days. I booked a fully refundable flight with the same airline at their office at the airport (thinking this would be the fastest way, so I don't miss my flight). Once in Thailand, I got the flight refunded, but the cancellation fee was higher than a cheap ticket would cost.

I was once asked at immigration to show a flight out of Thailand (because I have a year-long history of tourist visas) and was allowed to book a flight on the spot and show flight confirmation on my mobile phone. I suggest you just research flights and do the booking process on your phone until you get to payment, so you know how it works and can do it in the improbably case it should be required.