On my last crossing into Thailand via Poipet/Aranyaphrathet (US Passport, 30 plus years of entries and exits, prior retirement visa in Thailand, present retirement extension in Cambodia) I was not asked.
On my 2 or 3 Aranyaphrathet entries last year, I was not asked.
In fact in my entire life I have never been asked for an outward ticket by anyone other than Airline staff in the USA, and once on Air Asia when I was operating on Visa exempts. Long term Visas satisfy the airlines.
My kid was asked once at DMK Immigration on her first trip in and she looked like a bum. She had an outward ticket though
My friend from SA with a Cambodian Tourist Visa expiring that day was not asked in Aranyaphrathet, but she had at outward ticket back to Cambodia. She had one older Tourist Visa entry and exit.
When I was operating Visa exempt and needed a ticket (other than a real return), I would drop $50 on a ticket for something like Hat Yai to KL.