@David *******
you write "if people are asked to have an onward flight you would have to ask why each travellers history is different"...
That kind of answers the question! It varies, so the OP should have an onward ticket to present, if being asked.
I mean, just because you were never asked in 20 years travelling, does NOT mean they will not ask ANYONE (or even you in the future).
I have been to Thailand 50+ times and was never asked either... However, only a few times, did I travel with a oneway ticket. Meaning if you have return ticket on the same booking they see it in the system and they won't ask. If you travel with a visa they won't ask etc. It varies among the airlines too, some ask for it, some don't.
I believe that a persons travel history (when checking passports) can have some inpact as well, wether they ask to see a ticket or not (especially at immigration).
Being able to present an onward ticket is a formal request to enter Thailand, although immigration rarely asks. They can though... and they can deny entry.
That you and I were never asked to present one, will not help the OP if they are asked for it. "I read on facebook..." wont get them onboard the plane or through immigration.
😉
Hence I still advice to not travel with a oneway ticket.