There was a guy in NZ (Bruce Simpson) who detailed on his blog how to make a DIY cruise missile. I think he got politely asked to stop doing so at some point.

BPS.Space on youtube is working on a DIY space-capable rocket and in a recent video he mentioned that he is not doing this as a tutorial and that his guidance system likely already wanders into ITAR territory, and thus he's self-censoring which parts he shares and which parts happen off-camera.

Ultimately these kinds of regulations are fairly silly because a sufficiently determined smart person can recreate the covered technologies from scratch, but here we are.

Any sufficiently smart person can accomplish the same original work as any other sufficiently smart person.

But when the details of these things are published or otherwise made openly available, it doesn't take nearly as many smarts to duplicate these accomplishments.

Quite often, that's good: It's easy for a dullard like me to build a circuit or to re-use some clever assembler code when someone else has published it for my own tinkering around the house. In this way, it's a pretty great world to live in; it is often very simple to stand on the shoulders of giants and get some things done that I could probably never do on my own.

But sometimes, that's bad: We don't live in a perfect world. Enemies exist. Things like ITAR can't prevent a sufficiently smart person from doing anything, but they do make it a lot harder for them to get started.

Just don't let some future party use your code to do it.

Not if they buy an off-the-shelf solution.

correct. and the idea here is to put up at least some form of barrier to entry, because any good guidance is also nuclear warhead delivery guidance at a certain point.