Hm, haven't really thought about it. I usually use some old lines of code instead of researching how to do stuff all over again. Some people use extensive code librarys(e.g. DOTween) and depending on the engine the amount of stuff already in it varies greatly (e.g. Unity Monobehaviour, physics etc). When it comes to code you basically always use a pretty large amount of "assets" (if you want to call it an asset) unless you create your own engine from scratch.
TL;DR - I think generic code and features should be fine but distinguishing features/mechanics should always either be created in the jam or listed as an asset.