Have airlines consistently asked for proof of onward travel when flying to Thailand?

Yes, several times. I haven't been on visa exempt in years (since late 2013) but I have been asked in the following cases (this isn't an exhaustive list):

Ho Chi Minh City (Thai Airways), mid 2013. Explained I had a flight to Laos days later and was allowed to board the flight. A Vietnamese friend flying Lufthansa to Bangkok on a one way ticket was required to sign a liability waiver form such that if he was refused entry for not having a return flight the airline wouldn't have to take financial responsibility.

Sydney (Thai Airways), 2003: showed an outbound ticket to Phnom Penh. However, I did actually have a visa back then, but they still asked for proof of outbound travel!

2013? Sydney (Thai Airways), was asked for proof. Showed them my visa.

2012, Phnom Penh (Thai Airways). Verbal proof of outbound ticket sufficed.

Where I have NOT been asked for proof of outbound travel: Zurich (Thai Airways and Swiss) on multiple occasions dating back well over a decade.

Singapore (Singapore Airlines).

Vientiane (Thai Airways), multiple occasions.

Yangon and Mandalay (Thai and Thai Smile).

Vietjet

Never been asked but I’m guessing when you check in they can see if you’ve a return flight booked ?

for me I have always had a return or onward ticket but after hundreds of flights I was never asked. I have always entered from Europe or Singapore. It appears to me that North American, UK and Australian ask a lot maybe uptight Anglo Saxon

Yes, with Lufthansa

Air Europa.

Bottom line, have your poot.

Thai Airways (or Smile. I can't remember) Flying TO Phnom Penh and when RETURNING to Bangkok in 2022.

Air Canada in 2017 or 18 TO Bangkok.

Thai immigration 2 times.

Those are the ones I remember. I know there have been more.

Qatar is super strict

Emirates London to Trivandrum

Singapore Air but during Covid lockdowns from Melbounre.

Was refused by QR.

JAL

Air Canada, United, Swiss, EVA and ANA... basically Star Alliance. Asked departing Canada, departing the US and in transit.

It depends on whether or not you have a visa, and which visa you have.

Qatar Airlines...they let me book a hotel in Vietnam 30 days later which I cancelled when I arrived in Thailand

Ive never been asked but if my return ticket is with same airline then I guess they would already know. I've never been asked to show proof at immigration in Bangkok too, I just tell them when my return flight is if asked.

Sometimes a round trip ticket is cheaper.

Thai Airways

Thai Airways from Perth and also from Bali, had to produce itinerary so they could check onward travel each time

Been asked only once about 8 years ago

Jetstar

Air asia asked to see to my tourist visa this year

Emirates

Thai airways

Jetstar. MEL to BKK. Every time whenever I've booked a long stay and the check-in person needs me to show them the visa. Other times they don't ask because it's obvious from your booking - they can see that. So maybe ppl don't think it's being checked, but it certainly is. No airline wants to break that regulation 'cos they'll have the trouble of dealing with the consequences ie providing you with an outbound flight.

Since the Thai govt introduced the regulation.

Depends on your type of visa.

Yes everytime and i fly a lot Singapore or Thai airways

Always ask n Visa

Singapore Airlines (at Sydney) and Qatar (Berlin)

Air asia

United

Its the law on most short term visa's so the poll is a waste of time

Yes, China Airlines.

Qatar.

Air Canada and Qutar Airline.

We brought carry on luggage only, came over on qantas and was easy peasy getting through. No questions asked just scanned the passport and stamped.

Was Air NZ. Auckland to BKK