Of course I understand your posts! But, with all due respect, and not that I personally doubt you at all, but who's going to risk having their trip ruined because they've been denied boarding due to what an anonymous person who claims to be a check-in agent has posted on an internet forum? I'm trying to track any official confirmation out there to support what you're telling us.
So far, I haven't found anything I would regard as gold-standard, just negatives ie "proof of onward travel" no longer appears on the MFA website and on the internet pages of some embassies and I'm sure it was previously quite clear on the MFA site. But neither MFA or Immigration announced any change in the rules.
Maybe the rules have indeed been changed for some time, but airlines have still been clinging to the obsolete rule because the Thai government never advised them of the change? That would explain why some airlines have been denying boarding, but Thai Immigration anecdotally never checks for proof of onward travel.
The IATA database as accessed through Delta's website says:
"Additional Information:
- All passports must be in good condition.
- Immigration officers can request visitors who are visa exempt to show return/onward tickets and documents required for next destination."