[Sinapus]

If you save your empire, it will record whatever designs you have that are not obsolete, IIRC.

I found it handy to start a game at high level, then design some ships I tend to use (like colony ships) and save the empire. Then when I start a game with that empire it will be in there, just unbuildable until I get the right tech for it.

Pre 1.27, though, it had a nasty habit of making your ship designs vanish if you added other players into the game. Hopefully, something mentioned in the history files has fixed that problem.