The following is a post I made in February re onward tickets, etc.
"The old proof-of-exit/onward-travel/return-trip-required provision has been in effect for many decades now, beginning in the years following WWII. I began working with Pan Am in 1970, and during the next 15 years I was an employee of several other carriers, including Air France, Qantas, TWA. At all of these airlines, we were required to advise passengers that one-way tickets were permitted ONLY for citizens/ permanent residents of said passengers' destination countries, which meant that a non-citizen/non-resident customer needed to purchase a return or onward travel ticket prior to being issued a one-way ticket. As others have noted, this provision is upheld by either the carrier of transportation or the country of travel origin, or both. After leaving the airline industry, I worked in corporate travel for American Express in the US for 20 years, where this ticketing provision was enforced by company policy."
This is a general IATA policy that, nowadays, may or may not be enforced by some airlines and/or the destination country's immigration authorities, although the original rule remains in effect.