In the distant past, this was the homeworld of the presumed extinct Iconian species, located in what was, in the 24th century, the Romulan Neutral Zone. The planet was devastated by a massive orbital bombardment approximately 200,000 years ago. The Iconians were believed to have been annihilated, although a part of the population may have escaped through their gateways. By the 24th century, the planet had one working automated structure which launched probes at orbiting ships.
In 2256, its location was labeled on the star chart "Alpha/Beta Quadrant Overview" in the ready room aboard the USS Discovery. Although it was in Klingon space, it was not claimed by the Klingon Empire. (DIS: "Magic to Make the Sanest Man Go Mad")
The precise location of Iconia was lost to history until 2365, when an artifact bearing an ancient starmap was discovered in an archaeological dig on Denius III. From there, Captain Donald Varley of the Federation starship USS Yamato was able to locate Iconia, and chose to violate the Neutral Zone to reach the planet so as to prevent Iconian technology from falling into Romulan hands. After the Yamato was destroyed by an Iconian software transmission, the USS Enterprise-D and the Romulan warbird Haakona both traveled to Iconia, where Captain Jean-Luc Picard had the last operational structure on the surface destroyed. (TNG: "Contagion")
In 2372, Worf told the crew of the USS Defiant of the events that had taken place on Iconia when they were on a mission to destroy an Iconian gateway on Vandros IV in the Gamma Quadrant. (DS9: "To the Death")
According to the Star Trek Encyclopedia (4th ed., vol. 1, p. 356), the surface of Iconia was a matte painting designed by visual effects supervisor Dan Curry. The Iconian control building, a model used as part of the painting, was designed by Mike Okuda, based on two swimming pool filter covers and part of the Star Trek: The Motion Picture drydock model.
According to Star Trek: Star Charts (p. 63) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), Iconia's primary was a M-class star.
Michael Chabon, the former showrunner for Star Trek: Picard, named a Beta Quadrant sector after this planet in the blog article "Some Notes on Freecloud". Along with the Alpha Doradus system, which contained the inhabited planet Freecloud, this sector included the negotiated terminus of the Romulan Neutral Zone.