This came up when people started looking sideways at the ballooning costs of the Space Launch System. Even though we have extensive blueprints and even parts left over from Saturn V rockets, the institutional knowledge to use any of those things simply doesn't exist anymore, and would have to be re-created at exceptional cost of you wanted to build another one.

https://space.stackexchange.com/questions/6281/why-not-build...

IIRC, even if we could, we can do better. For example a Saturn V engine is around 5,000 parts, though we've lost the knowledge to assemble them. However, a contemporary study of that found that, with modern manufacturing techniques, you can build an equivalent today with just 40 parts.