Before, or after 1975?
Going back to topic, we need to first consider which cities would have the need to build airports in artificial islands. These are the first few cities that come to mind:
-Expansion of Tokyo International Airport, instead of building a new one in Narita
-New York
-Hong Kong (though the OTL new airport is an expansion of a pre-existing island)
-Singapore
-East London
-Leningrad
-San Francisco
-Miami
-Toronto
-Manila?
The problem with most of the other major cities in the world is that they do have enough land around them to build the airport. And even with the above cities, the only way you can get them to build an offshore airport (aside from Tokyo and Hong Kong which sort of already done that OTL) is if the urban pressure for land use could justify the costs of building an artificial island from scratch.