Just a note.
I just checked in at Frankfurt airport with China airlines to Bangkok. Since my return flight ist more then 45 away, the airline wanted prove of onward travel. I explained that I'll do a border run or similar. They called Bangkok immigration. I was told that in case the immigration officer at Bangkok airport wants to see a ticket, I can be refused entry. They gave me my boarding pass and I was good to go.
But, I booked a flight online to danang Vietnam straight away. Not to take any chances!
TLDR : Answer Summary
Many travelers have reported varying experiences with the requirement of providing an onward travel ticket when entering Thailand. While airlines often demand proof of onward travel prior to boarding, immigration officers in Thailand rarely check for this upon arrival, particularly for regular tourists. However, travelers are advised to have an onward ticket, as the airline might deny boarding if they cannot confirm your travel plans, due to potential liability for any passengers denied entry. Some have suggested booking fully refundable tickets or using frequent flyer miles as a precaution. Overall, while it's a common practice for airlines to request proof of onward travel, the actual enforcement by Thai immigration is inconsistent.