What word best describes a line of posts connected by a retractable belt?

Stanchions.

From https://en.m.wikipedia.org/wiki/Stanchion

Stanchions are used for many different purposes, including crowd control and waiting lines. Many different places use stanchions, including banks, stores, hotels, museums, restaurants, concert venues, trade shows, and other events.[citation needed]

Portable posts used to manage lines and queues.

Fixed posts with decorative ropes, custom printed belts, or metal wires. Often available in single, double, and triple belt/wire configurations.

Retractable belt stanchions, often with a slow retract belt mechanism for safety.

The last are the type you observed, common in airports.