The code they're looking for is already in the repo (just a version bump on some dependencies), so the response would actually have been "instructions for building from source can be found here [0]".
> The recommended way to install mitmproxy on Linux is to download the standalone binaries on mitmproxy.org.
> Dependencies in the binary packages are frozen on release, and can’t be updated in situ. This means that we necessarily capture any bugs or security issues that may be present. We don’t generally release new binary packages simply to update dependencies (though we may do so if we become aware of a really serious issue).
[0] https://docs.mitmproxy.org/stable/overview-installation/#adv...