This was way back in 1976. I don’t remember Japan, Hong Kong or any of the many SE Asian countries I visited actually checking for an onward ticket at customs. I just recall the people at Air Siam telling me so.
When we flew back from our visit to Hawaii 10 years ago, the China Airlines counter at Honolulu Airport wanted to see my return or onward ticket. When I explained this was my return ticket, I had to show them the Non-Immigrant Visa in my passport. I asked them what would happen if someone wanted to travel around the region and buy a ticket back home from somewhere else, and they said that person would have to purchase a round-trip ticket and then cash in the other half here in Thailand. That’s the only time that’s ever happened to me, but it was about that time I heard airlines were tightening up the rules to avoid getting stuck with having to fly passengers back who didn’t meet Immigration scrutiny. Immigration even then didn’t want to see an onward ticket or even proof of funds (another requirement).