It will always be possible to build small airports.
Configuration file: keepsmallairport
Command line: -k
With this option, you can always build small airports, even if you don't have any planes that can land there anymore. But at least helicopters can land safely, and it can hold more helicopters than a heliport, as well as being able to service them in the hangar.
Computer players do not get this benefit, because they aren't smart enough to use it properly. They'd build large planes and try to land them on small airports, causing an untold number of crashes. This option only applies to human players.
Note: In versions of TTDPatch earlier than 2.0 this option affected computer players as well as human players. In the games saved with those versions, computer players will retain the ability to build small airports after 1960 until the end of a month, or forever if the year is already 2050 or later.