Say you want a j1/m1 ship with civilian sensors. You don't need any high tech gear and are really leery of high mortgages. Do you buy the best that a TL 12 shipyard can put out because you can, or if you could, would you leverage the TL to get that TL9 ship at a discount, knowing that you can get TL12 parts for it at almost any port along your way?
It sort of has been brought up before. The example of an old radio. We aren't building an old radio, we are building an underperforming radio (by current standards) in the same form factor as an old one for less money.
Most likely you'd buy a used ship in that case, that's older, cheaper and fulfills all your needs. It would be very unlikely to go to a higher TL world and ask them to build down.
Think of it this way, you want a replica clipper ship (circa 1850s). It's got no machinery on it at all (OG design). You go to a shipyard and ask them to build it and they'd be like "umm, we don't work in lumber". Sourcing materials, finding labor who knows how to do it, etc, etc, would be impossible for commercial production. It COULD be done, but at great cost and time.
Same as the example of Liberty ships. Those were actual downgrades for their time, engine wise at least, in order to ensure the engine orders did not tie up more critically needed ones for warships. Building a modern Liberty ship today would get you the hull, but modern diesel, modern everything else.
Traveller game mechanics don't hold up well if you peer under the hood too much. The DC-3 is a fine workhorse of a plane and there are ones still flying today. But nobody has BUILT one in about 70 years. They fulfill a niche due to their simplicity and ruggedness - yet nobody builds them. Changes have been made to accommodate repairs for things you can't get today, but because of their simplicity they don't have glass cockpits either during retrofitting - though I'm sure some have modern GPS bolted somewhere in the cockpit as well as transponders tucked away somewhere to meet modern regulations.
In Traveller terms that'd be your TL12 ship built at TL15 shipyard. As a game, sure, we can do it. But if we want it to be a reflection on how things work in reality - yeah... no bueno there. Anytime you get into a complex piece of machinery, building things using the machinery and workforce experience of the current system that is available to you is the norm and everything else is a luxury/custom job.
I don't have my old Walkman anymore, but it was a radio / cassette player. Today I have a smartphone that's a computer/radio and can locally store thousands of songs - and the foot print is smaller. To build a Walkman today would be a custom job of a very experienced machinist since all the work would be done by hand and not on a production line using pre-manufactured components. Unless they could find all the parts sitting dusty on a shelf. Technology advances and everything else around it tends to do it as well.
I recall reading somewhere about an English car that became un-economical to produce in the UK, so they sold the car and the production machinery to India. Now people who still drive the cars import their parts from India. Can't recall if the car itself is still made or just the parts. That may be a better analogy for debate.