Between this question about deb/rpm and the one where you were asking how to install something directly from Github, I can’t help but feel that perhaps you are not asking the question you actually want answered. My sense is that despite the specifics of the questions you have posed, you don’t necessarily care about rpm/deb/github, but just about being able to run a piece of software.
If that’s the case, then there are many convenient technologies to run software through containerization or other means. A popular choice is Flatpak. If you are still trying to install Deskflow, that appears to be available on Flathub.
As much as declarative and reproducible is nice, it’s useful to know some of the escape hatches from nix / NixOS.