Nixpkgs is one of, if not the largest, package managers by # of packages. I think naturally that will result in more abandoned packages than others. What is the % of abandoned packages relative to total I can't say.
Nix tends to have updates almost as quickly as Arch does, I run a lot of bleeding edge software and have no issues, and the breadth of packages is immense. Nix just has a different style. With the AUR, you can adopt an abandoned package more easily. That has pros and cons (the AUR has recently suffered multiple supply-chain attacks due to packages being adopted and infected).
Nix tends to have updates almost as quickly as Arch does, I run a lot of bleeding edge software and have no issues, and the breadth of packages is immense. Nix just has a different style. With the AUR, you can adopt an abandoned package more easily. That has pros and cons (the AUR has recently suffered multiple supply-chain attacks due to packages being adopted and infected).