At my last job I spent a lot of time on cleanups and refactoring and never got the LLM to help me in any way. This is the thing that I try every few months and see what's changed, because one day it will be able to do the tedious things I need to get done and spare me the tedium.
Something I should try again is having the LLM follow a spec and see how it does. A long time ago I wrote some code to handle HTTP conditional requests. I pasted the standard into my code, and wrote each chunk of code in the same order as the spec. I bet the LLM could just do that for me; not a lot of knowledge of code outside that file was required, so you don't need many tokens of context to get a good result. But alas the code is already written and works. Maybe if I tried doing that today the LLM would just paste in the code I already wrote and it was trained on ;)
They are the quiet ones.