Oldtech refers to technologies which are obsolete or significantly behind the technological standards of the 'Verse. They are usually found in the hands of frontier colonists on the Rim planets or among the poorest elements of Border world society.
While more advanced technologies exist, and a wide range of tech levels are known in the 'Verse, the use of oldtech persists because such devices are generally cheap, easy to produce with primitive tools and limited resources, and do not require super-skilled craftsmen to build and maintain.
The Great Debate
There has been much criticism and debate in the fan community regarding the presence of Oldtech in a space-faring setting. Theories abound going beyond the obvious cinematic explanations of using old-style objects to set the Western mood, or using whatever the props department had on-hand at the time.
But there are many real-world historical examples of obsolete technologies being preferred even when superior technologies were available. The classic example of this is the Colt Peacemaker.
Colt Firearms produced the "New Model Army Metallic Cartridge Revolving Pistol" in 1873. It was a single-action, 6-shot revolver of basic design. By the standards of its day it was a primitive device, being easily a generation behind the firearm design standards of Europe and the American east coast. It was large, heavy, had to be cocked before each shot, was clumsy and slow to reload, and used a long-cased .45 cartridge that no other weapon in the US military arsenal could use.
But it rapidly became the weapon of choice in the American frontier. It was rugged and simple, inexpensive to make and easy to produce. It was durable enough that it could be dropped over a cliff without harm and would operate even if covered in mud and sand. It used only simple pins, levers and springs so that, if it did break, any skilled blacksmith could repair it. It could use either the standard military-issue short-case cartridge or the more powerful long-cased cartridge without modification. When specialty gun-oil wasn't available for maintenance, axle grease or cooking lard could be used just as well. For a primitive region without access to high-tech support services, it was the perfect weapon.
Such is the situation in the 'Verse where the Rim-world communities exist in relative isolation and must be self-sufficient. While advanced technologies are superior in many ways, the simpler "obsolete" technologies would also be in high demand.