Will I be okay at the airport with a 90-day return flight but only receiving a 60-day exception?

Always start with seeing what visa options there are, then decide how long you can stay.

The Airline normally determines but rarely asks if you from a country that has 60 days entry allowed. If they ask you, then say you will land cross into our of the neighbourhood countries. Last resort is a disposable ticket. I got mine for £15, which I never used. Don't stress over it. There are ways

I'm in Thailand now 60 days then extension 30 days easy

I'm no expert so can only share my experience...

I travelled to Chiang Mai last year on Qantas. I planned to stay 6 weeks, which was the time period of my outbound and return flights.

I emailed a visa agent in CM before my travel who gave free advice. She said I was fine to get a 30 day visa on arrival, and I could renew it for another 30 days at immigration in CM. This saved me the "hassle" (so I thought) of working with the Thai Consulate to have a visa period sufficient to cover my travel dates.

Qantas wouldn't let me on the outbound flight. Luckily (in this instance) the outbound flight was delayed 4 hours. I called the Thai Consulate, who emailed me a message saying they were ok for me to travel (I admit their email was a bit vague). Qantas still wouldn't let me on the flight even with the email.

My choices were limited: bail on the trip, at least for those dates, and likely lose all hotel deposits, or change the return flight to an earlier flight. Luckily there were earlier flights available. Immigration in BKK did not even ask about my return flight. Once in Thailand I moved my return flight back to my original date. Of course both changes incurred change fees.

Lesson learned: don't just consider what Thai immigration will do, but what the airline will do.

(As an aside, Qantas customer service is abominable. Literally 8 hours or longer on hold to speak to customer service - not an exaggeration although it may have improved since Alan Joyce got the boot; this question isn't covered by their website FAQ's; they don't provide email support; they don't answer questions in their FB group. Hopefully your airline makes it easier to get an answer to this scenario, but I would think the safest option is for your visa validity to completely cover the time period for your outbound and return flights. Otherwise make SURE you know what your airline will do when you're trying to check in.)

Hope this helps...

The rule is a little vague on this. Normally it makes no sense to ask your airline if you would be okay, because they don't make the rules and have no choice but to tell you that proof of onward travel is required, regardless of whether they'll enforce is, this is slightly different with this particular issue as now their interpretation of the rule comes into play. Normally, airlines will interpret the rule as that proof of onward travel needs to be within the original period that you get granted however, as it appears that this is also what the Thai authorities imply (although embassies, which also deal with the rule in case of tourist visa applications, often take it more leniently, and do account for extendability). So although the odds are very high that your airline upon consultation will tell you that proof of onward travel within 60 days is required (regardless of what the check-in crew will do), it could make sense to ask.

I was asked for my return ticket by Immigration in Thailand last February

You aren't asked by Immigration in Thailand, it's the check in at your airport of departure where you may be asked.

KLM wanted proof of visa

You should be fine but like Brandon recommended contact your airline and ask them

I came in on a 60-day tourist visa. My return flight is 84 days after arrival. I’ll get a 30-day extension when it gets close to the 60days in country and wasn’t asked by immigration

I have never been asked at immigration about details of my return flight, but my passport is nearly full of Thai entry and exit stamps, I never go anywhere else 😂

I was asked for the first time ever by Qatar at London Gatwick this month. Never been asked before in over 20+ visits. (I had a return ticket so no issue)

They never ask me for proof of leaving when I go there without a return ticket.

Just buy disposable onward ticket,

Google onward ticket about 12 quid each better safe than sorry

Asked at least 5 times per week.

ASK YOUR AIRLINE

They are the ones who might require you to show proof of leaving Thailand. And if they do then they will require it to be within 60 days, not 90.