Many game jams (e.g. GMTK) allow users to use a minimal amount of code which has been prepared beforehand (for example, I'd like to use SDL, and have created wrappers to handle keyboard input and UI stuff like bezier curves and text). There's nothing on the rules which mentions this (as far as I can tell).
I'm not talking about code which is designed/made for the game, just the basic stuff which other game engines (e.g. Unity) would already do for you.