You really misunderstand Nix then. Nix is the actual and only novel solution in this space — you can just have a single flake file in your repository to make i

You really misunderstand Nix then. Nix is the actual and only novel solution in this space — you can just have a single flake file in your repository to make it always reproducible. It will only build/download files that are necessary, using a minimum required space. But for this reason it can also just point you to a binary cache and you can copy the necessary data (basically just a diff of what is required dependency for running program P and your system). Just because the source is not available doesn’t mean that the dependencies can’t be explicitly specified and use some standard deps. Chances are that proprietary app will just depend on a libc you already have downloaded.