I'm looking for documentation about the old compiler frontend ( FE 1.0 ). Although it's much older than FIR, I can find only a few resources about it. Is there some internal documentation or does someone knows where to look for more information about the old frontend?

No docs at all. Everything community knows comes from trial and error plus conversations in this slack. We're all eagerly awaiting for K2 stable which should bring stable compiler plugin api and docs

Wouldn't it make sense to collect these drial and error experiences in a community-made documentation?

It would but that requires someone to spend time and effort

Ok, fine - I do it.