@Phillip *******
If you are flying out without a visa or on a one-way flight, you must expect that the airline CAN ask you for proof of onward travel from the destination country within the expected length of stay stamped upon entry.
They "can", they don't have to. But if the check-in crew consistently implements the aviation rules, they will definitely ask for it. Because this rule EXISTS!
If you have never been asked for the onward travel proof when flying to Thailand without a visa, this means you have been lucky
Thai immigration is usually not interested in proof of onward travel or the return flight. They only ask for it randomly.
It is also important to know that many airlines do NOT accept a train or bus ticket to a neighboring country as proof of onward travel, but insist that it must be a FLIGHT ticket.
since a bus only goes to the border and does not cross it (right-hand drive/left-hand drive!), such tickets are sometimes NOT sufficient as proof
Of course, there are people in every group who claim the opposite, but they can never GUARANTEE it, whereas a flight ticket is always a 100% guarantee
Then there are people who firmly claim that the proof of onward travel is not true, just because they themselves have never been asked for it.
But these people can never guarantee in these groups that this will not happen. That is why such behavior is irresponsible and dangerous!
There is NO guarantee that you will not be asked at check-in. It could just as easily be the other way around and proof of onward travel will be requested.
Then you stand at check-in and cannot board!
That is why I do not think it is a good idea to present your own experience as irrefutable. Thousands of travelers have already been turned away in the past.