Does this work for release builds or just debug builds now?

It only works with our self-hosted code generation backends (the main one being for x86_64), which right now don't have any optimisation passes. It's planned that they will in future, but that's a long-term goal. Also, any optimisations which propagate information between functions (the most obvious and important one is inlining) are more-or-less incompatible with incremental compilation (I touch on this in the post IIRC), so once this does work it'll probably still be limited to a subset of optimisations.

TL;DR: only debug builds for now, could extend to release builds once we have our own optimisation passes one day, but some optimisations will still be inapplicable.

I don't think it will work with llvm/release yet but it might some day maybe.

Getting incremental linker to work with llvm is kinda hard atm. Maybe the zig team has a plan dunno.