I think it's fair to posit that the standard blueprint package is probably a living document. Likely maintained by an Imperial Standards Bureau that is itself the continuation of an original Vilani one.
It does not mean at all that the designs are still TL9. But new innovations that make a difference (including hull materials, software and demand for a type) will only pass into the standards gradually, after thorough testing and review.
And it's not a stretch to suggest every civilian shipyard is built on the assumption that it's going to be making ships to these standards. Does not mean the standards are overly restrictive - there's likely several standards for a hatch or an iris valve, and interior design work probably falls outside the scope - but they're all working from the same designs in yards that were BUILT from the shipyard design standards, using parts and components that were themselves made from the standards.
It might be worth thinking of standard starship production as being more like aircraft production than watercraft production. Water ships have a lot more leeway in terms of capacity and exact dimensions. Especially the way things like container ships are built, adding or removing an extra hull segment during the build won't require a complete regime of modelling and testing to see if the thing can fly.
(Edit: In fact, spaceship design and construction is of course a real world thing and is indeed far more like aircraft design and construction. Unsurprising, since our admittedly primitive spacecraft have to deal with atmosphere as well as orbit. But then, Traveller ones have to deal with all that and different gravity and different atmospheres and Jump Space and interplanetary flight)
